6-K

U.S. SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16 UNDER THE

SECURITIES EXCHANGE ACT OF 1934

Dated October 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: October 30, 2025

Exhibit 99.1

GERDAU S.A.

Tax ID (CNPJ/ME) 33.611.500/0001-19

Registry (NIRE): 35300520696

NOTICE TO THE MARKET

GERDAU S.A. (B3: GGBR / NYSE: GGB) (“Company”) informs that its subsidiary GUSAP III LP has been authorized to negotiate all terms, execute all related documents and amendments, and take all necessary actions for the redemption of the totality of the Bonds issued, with a principal amount of US$500,000,000.00, an interest rate of 4.25%, and maturity in 2030.

The redemption, also known as the “Make-Whole”, will be made pursuant to Section 3.1(b) of the Indenture. The Company also informs that bondholders were sent today the notice of redemption, and that the redemption date is expected to be December 2, 2025. The redemption is being made solely pursuant to the notice of redemption dated as of today, and this notice to the market does not constitute a notice of redemption of the Bonds.

São Paulo, October 30, 2025.

Rafael Dorneles Japur

Vice-President and

Investor Relations Officer