UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K ☒ Annual Report Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the fiscal year ended December 31, 2023 ☐ Transition Report Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the transition period from to Commission file number 1-09761 ARTHUR J. GALLAGHER & CO. (Exact name of registrant as specified in its charter) DELAWARE 36-2151613 (State or other jurisdiction of (I.R.S. Em ...