Gateway Trust N-CSRSCertified semi-annual shareholder report (management investment company)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-CSR

CERTIFIED SHAREHOLDER REPORT OF REGISTERED

MANAGEMENT INVESTMENT COMPANIES

Investment Company Act file number: 811-22099

Gateway Trust

(Exact name of Registrant as specified in charter)

888 Boylston Street, Suite 800 Boston, Massachusetts 02199-8197

(Address of principal executive offices) (Zip code)

Susan McWhan Tobin, Esq.

Natixis Distribution, LLC

888 Boylston Street, Suite 800

Boston, Massachusetts 02199-8197

(Name and address of agent for service)

Registrant’s telephone number, including area code: (617) 449-2139

Date of fiscal year end: December 31

Date of reporting period: June 30, 2023

Item 1. Reports to Stockholders.

(a) The Registrant’s semi-annual report transmitted to shareholders pursuant to Rule 30e-1 under the Investment Company Act of 1940 is as follows:

Semiannual Report

June 30, 2023

Gateway Fund

Gateway Equity Call Premium Fund

Mirova Global Green Bond Fund

Mirova Global Sustainable Equity Fund

Mirova International Sustainable Equity Fund

Mirova U.S. Sustainable Equity Fund

Table of Contents

GATEWAY FUND

Investment Goal

The Fund seeks to capture the majority of returns associated with equity market investments, while exposing investors to less risk than other equity investments.

Average Annual Total Returns – June 30, 2023⁴

Performance data shown represents past performance and is no guarantee of, and not necessarily indicative of, future results. Total return and value will vary, and you may have a gain or loss when shares are sold. Current performance may be lower or higher than quoted. For more recent month-end performance, visit im.natixis.com/performance. Performance for other share classes will be greater or less than shown based on differences in fees and sales charges. You may not invest directly in an index. Performance for periods less than one year is cumulative, not annualized. Returns reflect changes in share price and reinvestment of dividends and capital gains, if any. The table(s) do not reflect taxes shareholders might owe on any fund distributions or when they redeem their shares.

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GATEWAY EQUITY CALL PREMIUM FUND

Investment Goal

The Fund seeks total return with less risk than U.S. equity markets.

Average Annual Total Returns – June 30, 2023⁴

Performance data shown represents past performance and is no guarantee of, and not necessarily indicative of, future results. Total return and value will vary, and you may have a gain or loss when shares are sold. Current performance may be lower or higher than quoted. For most recent month-end performance, visit im.natixis.com/performance. Performance for other share classes will be greater or less than shown based on differences in fees and sales charges. You may not invest directly in an index. Performance for periods less than one year is cumulative, not annualized. Returns reflect changes in share price and reinvestment of dividends and capital gains, if any. The table(s) do not reflect taxes shareholders might owe on any fund distributions or when they redeem their shares.

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MIROVA GLOBAL GREEN BOND FUND

Investment Goal

The Fund seeks to provide total return, through a combination of capital appreciation and current income, by investing in green bonds.

Average Annual Total Returns – June 30, 2023²

Performance data shown represents past performance and is no guarantee of, and not necessarily indicative of, future results. Total return and value will vary, and you may have a gain or loss when shares are sold. Current performance may be lower or higher than quoted. For most recent month-end performance, visit im.natixis.com/performance. Performance for other share classes will be greater or less than shown based on differences in fees and sales charges. You may not invest directly in an index. Performance for periods less than one year is cumulative, not annualized. Returns reflect changes in share price and reinvestment of dividends and capital gains, if any. The table(s) do not reflect taxes shareholders might owe on any fund distributions or when they redeem their shares.

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MIROVA GLOBAL SUSTAINABLE EQUITY FUND

Investment Goal

The Fund seeks long-term capital appreciation.

Average Annual Total Returns – June 30, 2023³

Performance data shown represents past performance and is no guarantee of, and not necessarily indicative of, future results. Total return and value will vary, and you may have a gain or loss when shares are sold. Current performance may be lower or higher than quoted. For most recent month-end performance, visit im.natixis.com/performance. Performance for other share classes will be greater or less than shown based on differences in fees and sales charges. You may not invest directly in an index. Performance for periods less than one year is cumulative, not annualized. Returns reflect changes in share price and reinvestment of dividends and capital gains, if any. The table(s) do not reflect taxes shareholders might owe on any fund distributions or when they redeem their shares.

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MIROVA INTERNATIONAL SUSTAINABLE EQUITY FUND

Investment Goal

The Fund seeks long-term capital appreciation.

Average Annual Total Returns – June 30, 2023²

Performance data shown represents past performance and is no guarantee of, and not necessarily indicative of, future results. Total return and value will vary, and you may have a gain or loss when shares are sold. Current performance may be lower or higher than quoted. For most recent month-end performance, visit im.natixis.com/performance. Performance for other share classes will be greater or less than shown based on differences in fees and sales charges. You may not invest directly in an index. Performance for periods less than one year is cumulative, not annualized. Returns reflect changes in share price and reinvestment of dividends and capital gains, if any. The table(s) do not reflect taxes shareholders might owe on any fund distributions or when they redeem their shares.

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MIROVA U.S. SUSTAINABLE EQUITY FUND

Investment Goal

The Fund seeks long-term capital appreciation.

Average Annual Total Returns – June 30, 2023³

Performance data shown represents past performance and is no guarantee of, and not necessarily indicative of, future results. Total return and value will vary, and you may have a gain or loss when shares are sold. Current performance may be lower or higher than quoted. For most recent month-end performance, visit im.natixis.com/performance. Performance for other share classes will be greater or less than shown based on differences in fees and sales charges. You may not invest directly in an index. Performance for periods less than one year is cumulative, not annualized. Returns reflect changes in share price and reinvestment of dividends and capital gains, if any. The table(s) do not reflect taxes shareholders might owe on any fund distributions or when they redeem their shares.

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ADDITIONAL INFORMATION

All investing involves risk, including the risk of loss. There is no assurance that any investment will meet its performance objectives or that losses will be avoided.

ADDITIONAL INDEX INFORMATION

This document may contain references to third party copyrights, indexes, and trademarks, each of which is the property of its respective owner. Such owner is not affiliated with Natixis Investment Managers or any of its related or affiliated companies (collectively “Natixis Affiliates”) and does not sponsor, endorse or participate in the provision of any Natixis Affiliates services, funds or other financial products.

The index information contained herein is derived from third parties and is provided on an “as is” basis. The user of this information assumes the entire risk of use of this information. Each of the third party entities involved in compiling, computing or creating index information disclaims all warranties (including, without limitation, any warranties of originality, accuracy, completeness, timeliness, non-infringement, merchantability and fitness for a particular purpose) with respect to such information.

PROXY VOTING INFORMATION

A description of the Natixis Funds’ proxy voting policies and procedures is available without charge, upon request, by calling Natixis Funds at 800-225-5478; on the Natixis Funds’ website at im.natixis.com; and on the Securities and Exchange Commission’s (“SEC’s”) website at www.sec.gov. Information regarding how the Natixis Funds voted proxies relating to portfolio securities during the most recent 12-month period ended June 30 is available from the Natixis Funds’ website and the SEC’s website.

QUARTERLY PORTFOLIO SCHEDULES

The Natixis Funds file a complete schedule of portfolio holdings with the SEC for the first and third quarters of each fiscal year as an exhibit to its reports on Form N-PORT. The Funds’ Form N-PORT reports are available on the SEC’s website at www.sec.gov. First and third quarter schedules of portfolio holdings are also available at im.natixis.com/funddocuments. A hard copy may be requested from the Fund at no charge by calling 800-225-5478.

CFA^® and Chartered Financial Analyst^® are registered trademarks owned by the CFA Institute.

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UNDERSTANDING FUND EXPENSES

As a mutual fund shareholder, you incur different costs: transaction costs, including sales charges (loads) on purchases and contingent deferred sales charges on redemptions, and ongoing costs, including management fees, distribution and/or service fees (12b-1 fees), and other fund expenses. Certain exemptions may apply. These costs are described in more detail in the Funds’ prospectus. The following examples are intended to help you understand the ongoing costs of investing in the Funds and help you compare these with the ongoing costs of investing in other mutual funds.

The first line in the table of each class of Fund shares shows the actual account values and actual Fund expenses you would have paid on a $1,000 investment in the Fund from January 1, 2023 through June 30, 2023. To estimate the expenses you paid over the period, simply divide your account value by $1,000 (for example $8,600 account value divided by $1,000 = 8.60) and multiply the result by the number in the Expenses Paid During Period column as shown for your class.

The second line in the table for each class of fund shares provides information about hypothetical account values and hypothetical expenses based on the Fund’s actual expense ratios and an assumed rate of return of 5% per year before expenses, which is not the Fund’s actual return. The hypothetical account values and expenses may not be used to estimate the actual ending account balance or expenses you paid on your investment for the period. You may use this information to compare the ongoing costs of investing in the Fund and other funds. To do so, compare this 5% hypothetical example with the 5% hypothetical examples that appear in the shareholder reports of the other funds.

Please note that the expenses shown reflect ongoing costs only, and do not include any transaction costs, such as sales charges. Therefore, the second line in the table of each fund is useful in comparing ongoing costs only, and will not help you determine the relative costs of owning funds. If transaction costs were included, total costs would be higher.

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BOARD APPROVAL OF ADVISORY AGREEMENT

The Board of Trustees of the Trusts (the “Board”), including the Independent Trustees, considers matters bearing on each Fund’s advisory agreement (collectively, the “Agreements”) at most of its meetings throughout the year. Each year, usually in the spring, the Contract Review Committee of the Board meets to review the Agreements to determine whether to recommend that the full Board approve the continuation of the Agreements, typically for an additional one-year period. This meeting typically includes all the Independent Trustees, including the Trustees who do not serve on the Contract Review Committee. After the Contract Review Committee has made its recommendation, the full Board, including the Independent Trustees, determines whether to approve the continuation of the Agreements at its June Board meeting.

In connection with these meetings, the Trustees receive materials that the Funds’ investment advisers (the “Advisers”) believe to be reasonably necessary for the Trustees to evaluate the Agreements. These materials generally include, among other items, (i) information on the investment performance of the Funds and the performance of peer groups of funds and the Funds’ performance benchmarks, (ii) information on the Funds’ advisory fees and other expenses, including information comparing the Funds’ advisory fees to the fees charged to institutional accounts with similar strategies managed by the Advisers, if any, and to those of peer groups of funds and information about any applicable expense limitations and/or fee “breakpoints,” (iii) sales and redemption data in respect of the Funds, (iv) information about the profitability of the Agreements to the Advisers, including how profitability is determined for the Funds, and (v) information obtained through the completion by the Advisers of a questionnaire distributed on behalf of the Trustees. The Board, including the Independent Trustees, also considers other matters such as (i) each Fund’s investment objective and strategies and the size, education and experience of the Advisers’ respective investment staffs and their use of technology, external research and trading cost measurement tools, (ii) arrangements in respect of the distribution of the Funds’ shares and the related costs, (iii) the allocation of the Funds’ brokerage, if any, including, to the extent applicable, allocations to brokers affiliated with the Advisers and the use of “soft” commission dollars to pay for research and other similar services, (iv) each Adviser’s policies and procedures relating to, among other things, compliance, trading and best execution, proxy voting, liquidity and valuation, (v) information about amounts invested by the Funds’ portfolio managers in the Funds or in similar accounts that they manage and (vi) the general economic outlook with particular emphasis on the mutual fund industry. Throughout the process, the Trustees are afforded the opportunity to ask questions of and request additional materials from the Advisers and the Independent Trustees meet separately with independent legal counsel outside the presence of Adviser personnel.

In addition to the materials requested by the Trustees in connection with their annual consideration of the continuation of the Agreements, the Trustees receive materials in advance of each regular quarterly meeting of the Board that provide detailed information about the Funds’ investment performance and the fees charged to the Funds for advisory and other services. The information received by the Trustees generally includes, where available, among other things, an internal performance rating for each Fund based on agreed-upon criteria, graphs showing each Fund’s performance and expense differentials against each Fund’s peer group/category of funds, total return information for various periods, performance rankings provided by a third-party data provider for various periods comparing a Fund against similarly categorized funds, and performance ratings provided by a different third-party rating organization. The portfolio management team for each Fund or other representatives of the Advisers make periodic presentations to the Contract Review Committee and/or the full Board, and Funds identified as presenting possible performance concerns may be subject to more frequent Board or Committee presentations and reviews. In addition, the Trustees are periodically provided with detailed statistical information about each Fund’s portfolio. The Trustees also receive periodic updates between meetings, both at the Board and at the Committee level.

The Board most recently approved the continuation of the Agreements for a one-year period at its meeting held in June 2023. In considering whether to approve the continuation of the Agreements, the Board, including the Independent Trustees, did not identify any single factor as determinative. Individual Trustees may have evaluated the information presented differently from one another, giving different weights to various factors. Matters considered by the Trustees, including the Independent Trustees, in connection with their approval of the Agreements included, but were not limited to, the factors listed below.

The nature, extent and quality of the services provided to the Funds under the Agreements. The Trustees considered the nature, extent and quality of the services provided by the Advisers and their affiliates to the Funds and the resources dedicated to the Funds by the Advisers and their affiliates. The Trustees also considered their experience with other funds advised by the Advisers, as well as the affiliation between the Advisers and Natixis Investment Managers, LLC, whose affiliates provide investment advisory services to other funds in the Natixis family of funds.

The Trustees considered not only the advisory services provided by the Advisers to the Funds, but also the benefits to the Funds from the monitoring and oversight services provided by Natixis Advisors, LLC (“Natixis Advisors”). They also considered the administrative and shareholder services provided by Natixis Advisors and its affiliates to the Funds. They also took into consideration increases in the services provided resulting from new regulatory requirements, such as recent rules relating to the fair valuation of investments and the use of derivatives, as well as from monitoring proposed rules, such as those relating to privacy and cybersecurity, environmental, social and governance-specific disclosures, and vendor oversight.

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For each Fund, the Trustees also considered the benefits to shareholders of investing in a mutual fund that is part of a family of funds that offers shareholders the right to exchange shares of one type of fund for shares of another type of fund, and provides a variety of fund and shareholder services.

After reviewing these and related factors, the Trustees concluded, within the context of their overall conclusions regarding each of the Agreements, that the nature, extent and quality of services provided supported the renewal of the Agreements.

Investment performance of the Funds and the Advisers. As noted above, the Trustees received information about the performance of the Funds over various time periods, including information that compared the performance of the Funds to the performance of peer groups and categories of funds and the Funds’ respective performance benchmarks. The Board noted that while it found the data provided by the independent third-party data provider useful, it recognized its limitations, including, in particular, that notable differences may exist between the Funds and the performance comparisons (for example, with respect to investment strategies) and that the results of the performance comparisons may vary depending on (i) the end dates for the performance periods that were selected and (ii) the selection of the performance comparisons. The Trustees also received information about how comparative peer groups are constructed. In addition, the Trustees reviewed data prepared by an independent third-party rating organization that analyzed the performance of the Funds using a variety of performance metrics, including metrics that measured the performance of the Funds on a risk adjusted basis.

The Board noted that, through December 31, 2022, each Fund’s one-, three- and five-year performance, as applicable, stated as percentile rankings within categories selected by the independent third-party data provider, was as follows (where the best performance would be in the first percentile of its category):

In the case of each Fund that had performance that lagged that of a relevant category median as determined by the independent third-party data provider for certain (although not necessarily all) periods, the Board concluded that other factors relevant to performance supported renewal of the Agreements. These factors included one or more of the following: (1) that the underperformance was attributable, to a significant extent, to investment decisions (such as security selection or sector allocation) by the Adviser that were reasonable and consistent with the Fund’s investment objective and policies; (2) that the majority of the Funds in the Fund’s category do not have a comparable mandate and so have a significantly different investible universe; (3) that the Fund’s longer-term (three-year and five-year) performance was stronger relative to its category; (4) that the Fund had outperformed its relevant benchmark for the one-year period ended December 31, 2022; (5) relatively recent changes to the composition of the Fund’s portfolio management team; (6) that the Fund’s long-term (10-year) performance remains strong relative to its category; and (7) that the Fund’s performance had been consistent with its investment objective, such that its performance relative to its category would be expected to lag in certain market conditions. The Board also considered information about the Funds’ more recent performance, including how performance over various periods had been impacted by various factors such as market and economic events.

The Trustees also considered each Adviser’s performance and reputation generally, the performance of the fund family generally, and the historical responsiveness of the Advisers to Trustee concerns about performance and the willingness of the Advisers to take steps intended to improve performance.

After reviewing these and related factors, the Trustees concluded, within the context of their overall conclusions regarding each of the Agreements, that the performance of the Funds and the Advisers and/or other relevant factors supported the renewal of the Agreements.

The costs of the services to be provided and profits to be realized by the Advisers and their affiliates from their respective relationships with the Funds. The Trustees considered the fees charged to the Funds for advisory and administrative services as well as the total expense levels of the Funds. This information included comparisons (provided both by management and by an independent third party) of the Funds’ advisory fees and total expense levels to those of their peer groups and information about the advisory fees charged by the Advisers to comparable accounts (such as institutional separate accounts), as well as information about differences in such fees and the reasons for any such differences. In considering the fees charged to comparable accounts, the Trustees considered, among other things, management’s representations about the differences between managing mutual funds as compared to other types of accounts, including the additional resources required to effectively manage mutual fund assets, the greater regulatory costs associated with the management of such assets, and the entrepreneurial, regulatory and other risks associated with sponsoring and managing mutual funds. In evaluating each Fund’s advisory fees, the Trustees also took into account the demands, complexity and quality of the

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investment management of such Fund, and the need for the Advisers to offer competitive compensation and the potential need to expend additional resources to the extent the Fund grows in size. The Trustees considered that over the past several years, management had demonstrated its intention to have competitive fee levels by making recommendations regarding reductions in advisory fee rates, implementation of advisory fee breakpoints and the institution of advisory fee waivers and expense limitations for various funds in the fund family. They noted that all of the Funds included have expense limitations in place, and they considered the amounts waived or reimbursed by the Adviser for all of the Funds under their expense limitation agreements. The Trustees also noted that the total advisory fee rate for each Fund was at or below the median of its peer group of funds. The Board also considered that the fee and expense information reflected information as of a certain date and that historical asset levels may differ from current asset levels, particularly in a period of market volatility.

The Trustees also considered the compensation directly or indirectly received by the Advisers and their affiliates from their relationships with the Funds. The Trustees reviewed information provided by management as to the profitability of the Advisers’ and their affiliates’ relationships with the Funds, and information about how expenses are determined and allocated for purposes of profitability calculations. They also reviewed information provided by management about the effect of distribution costs and changes in asset levels on Adviser profitability, including information regarding resources spent on distribution activities. When reviewing profitability, the Trustees also considered information about court cases in which adviser compensation or profitability were issues, the performance of the relevant Funds, the expense levels of the Funds, whether the Advisers had implemented breakpoints and/or expense limitations with respect to such Funds and the overall profit margin of Natixis Investment Managers, LLC compared to that of certain other investment managers for which such data was available. The Board also noted the competitive nature of the global asset management industry.

After reviewing these and related factors, the Trustees concluded, within the context of their overall conclusions regarding each of the Agreements, that the advisory fees charged to each of the Funds were fair and reasonable, and that the costs of these services generally and the related profitability of the Advisers and their affiliates in respect of their relationships with the Funds supported the renewal of the Agreements.

Economies of Scale. The Trustees considered the existence of any economies of scale in the provision of services by the Advisers and whether those economies are shared with the Funds through breakpoints in their investment advisory fees or other means, such as expense limitations. The Trustees also considered management’s explanation of the factors that are taken into account with respect to the implementation of breakpoints in investment advisory fees or expense limitations, which reduced the total expenses borne by shareholders. With respect to economies of scale, the Trustees noted that Gateway Fund had breakpoints in its advisory fees and that each of the Funds was subject to an expense limitation. In considering these issues, the Trustees also took note of the costs of the services provided (both on an absolute and on a relative basis) and the profitability to the Advisers and their affiliates of their relationships with the Funds, as discussed above. The Trustees also considered that the Funds have benefitted from the substantial reinvestment certain Advisers had made into their businesses.

After reviewing these and related factors, the Trustees concluded, within the context of their overall conclusions regarding each of the Agreements, that the extent to which economies of scale were shared with the Funds supported the renewal of the Agreements.

The Trustees also considered other factors, which included but were not limited to the following:

Based on their evaluation of all factors that they deemed to be material, including those factors described above, and assisted by the advice of independent counsel, the Trustees, including the Independent Trustees, concluded that each of the existing Agreements should be continued through June 30, 2024.

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LIQUIDITY RISK MANAGEMENT PROGRAM

Annual Report for the Period Commencing on January 1, 2022 and ending December 31, 2022 (including updates through June 30, 2023)

Effective December 1, 2018 (December 28, 2018 for Mirova International Sustainable Equity Fund and December 15, 2020 for Mirova U.S. Sustainable Equity Fund), the Funds adopted a liquidity risk management program (the “Program”) pursuant to the requirements of Rule 22e-4 under the Investment Company Act of 1940, as amended (the “Rule”). The Rule requires registered open-end funds, including mutual funds and exchange-traded funds, to establish liquidity risk management programs in order to effectively manage fund liquidity and mitigate the risk that a fund could not meet redemption requests without significantly diluting the interests of remaining investors.

The Rule requires the Funds to assess, manage and review their liquidity risk considering applicable factors during normal and foreseeable stressed conditions. In fulfilling this requirement, each Fund assesses and reviews (where applicable and amongst other matters) its investment strategy, portfolio holdings, possible investment concentrations, use of derivatives, short-term and long-term cash flow projections, use of cash and cash equivalents, as well as borrowing arrangements and other funding sources. Each Program has established a Program Administrator (“Administrator”) which is the adviser of the Fund.

In accordance with the Program, each of the Fund’s portfolio investments is classified into one of four liquidity categories based on a determination of a reasonable expectation for how long it would take to convert the investment to cash (or sell or dispose of the investment) without significantly changing its market value.

Each Fund is prohibited from acquiring an investment if, after the acquisition, its holdings of illiquid assets will exceed 15% of its net assets. If a Fund does not hold a majority of highly liquid investments in its portfolio, then the Fund is required to establish a highly liquid investment minimum (“HLIM”). None of the Funds has established an HLIM.

During the period from January 1, 2022 to December 31, 2022, there were no material changes to the Programs and no material events that impacted the operation of the Funds’ Programs. During the period, the Funds held sufficient liquid assets to meet redemptions on a timely basis and did not have any illiquid security violations.

During the period January 1, 2023 through June 30, 2023, the Funds held sufficient liquid assets to meet redemptions on a timely basis and did not have any illiquid security violations.

Annual Program Assessment and Conclusion

In the opinion of the Program Administrators, the Program of each Fund approved by the Funds’ Board is operating effectively. The Program Administrators have also monitored, assessed and managed each Fund’s liquidity risk regularly throughout the period.

Pursuant to the Rule’s requirements, the Board has received and reviewed a written report prepared by each Fund’s Program Administrator that addressed the operation of the Programs, assessed their adequacy and effectiveness and described any material changes made to the Programs.

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Portfolio of Investments – as of June 30, 2023 (Unaudited)

Gateway Fund

See accompanying notes to financial statements.

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Portfolio of Investments – as of June 30, 2023 (Unaudited)

Gateway Fund – (continued)