6-K

U.S. SECURITIESAND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

REPORT OF FOREIGNPRIVATE ISSUER

PURSUANT TO RULE13a-16 OR 15d-16 UNDER THE

SECURITIES EXCHANGE ACT OF 1934

Dated April 30, 2025

Commission File Number 1-14878

GERDAU S.A.

(Translation of Registrant’s Name into English)

Av. Dra. Ruth Cardoso, 8,501 – 8° andar

São Paulo, São Paulo - Brazil CEP 05425-070

(Address of principal executive offices)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F.

Form 20-F x        Form 40-F ¨

Exhibit Index

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this Report to be signed on its behalf by the undersigned, thereunto duly authorized.

Date: April 30, 2025

Exhibit 99.1

GERDAU S.A.

Corporate Tax ID (CNPJ/MF): 33.611.500/0001-19

Registry (NIRE): 35300520696

NOTICE TO THE MARKET

Gerdau S.A. (B3: GGBR / NYSE: GGB) ("Company"), following up on the Notice to the Market released on May 03, 2024, informs its shareholders and the market in general that it decided to end the feasibility analysis of a new greenfield unit in Mexico, whose focus would be on the production of special steels (SBQ), mainly aiming to serve the automotive sector. This decision is the result of a strategic reassessment by Gerdau in the face of the current international scenario, marked by an increase in trade defense measures adopted by several countries, in addition to the growing uncertainties regarding the future dynamics of the global automotive chain.

The Company will continue to closely monitor market conditions and remains committed to pursuing opportunities in line with its long-term strategy.

São Paulo, April 29, 2025.

Rafael Dorneles Japur

Vice-President and Investor Relations Officer