Company: GANX
Filing Date: 2025-11-28
Form Type: 424B5
Source: 0001104659-25-116944
Chunk: 1

Company: Gain Therapeutics, Inc.
Filing Date: 2025-11-28
Form: 424B5
Chunk 1
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 securities, but will act as our sales agent, using commercially reasonable efforts to sell on our behalf all of the shares of common stock requested to be sold by us, consistent with its normal trading and sales practices, on mutually agreed terms between Oppenheimer and us. There is no arrangement for funds to be received in any escrow, trust or similar arrangement.

The compensation to Oppenheimer for sales of common stock sold pursuant to the Distribution Agreement will be an amount equal to 3.0% of the gross proceeds of any shares of common stock sold under the Distribution Agreement. See “ Plan of Distribution ” for additional information regarding the compensation to be paid to Oppenheimer. The net proceeds from any sales under this prospectus supplement will be used as described under “ Use of Proceeds .” The proceeds we receive from sales of our common stock, if any, will depend on the number of shares actually sold and the offering price of such shares.

In connection with the sale of the common stock on our behalf, Oppenheimer will be deemed to be an “underwriter” within the meaning of the Securities Act, and the compensation of Oppenheimer will be deemed to be underwriting commissions or discounts. We have also agreed to provide indemnification and contribution to Oppenheimer with respect to certain liabilities, including liabilities under the Securities Act or the Securities Exchange Act of 1934, as amended, or the Exchange Act.

Investing in our securities involves a high degree of risk. You should review carefully the risks and uncertainties described under the heading “Risk Factors” on page S-7of this prospectus supplement and under similar headings in the other documents that are incorporated by reference into this prospectus supplement.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement is truthful or complete. Any representation to the contrary is a criminal offense.

#### Oppenheimer & Co.
<div align='center'>The date of this prospectus supplement is November 28, 2025.</div>

TABLE OF CONTENTS

TABLE OF CONTENTS Prospectus Supplement

| ​ | ABOUT THIS PROSPECTUS SUPPLEMENT                  | ​ | ​ | ​ | ​ | S-1  | ​ | ​ |
| ​ | SUMMARY                                           | ​ | ​ | ​ | ​ | S-3  | ​ | ​ |
| ​ | RISK FACTORS                                      | ​ | ​ | ​ | ​ | S-7  | ​ | ​ |
| ​ | SPECIAL NOTE