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 1, 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

Exhibit	Description of Exhibit

99.1	Material Fact, “CAPEX Investment Plan”, October 1, 2025

99.2	Material Fact, October 1, 2025

99.3	Gerdau Investor Day 2025

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 1, 2025

	GERDAU S.A.

	By:	/s/ Rafael Dorneles Japur
	Name:	Rafael Dorneles Japur
	Title:	Executive Vice President and Investor Relations Director

GERDAU S.A. 6-K