Is GTA 6 rated M for Mature?

Yes. The ESRB rated GTA 6 M for Mature 17+, for Blood and Gore, Intense Violence, Mature Humor, Nudity, Strong Language, Strong Sexual Content, and Use of Drugs and Alcohol, with In-Game Purchases. Rockstar's site and both US console stores show it. It is PEGI 18 in the UK and R 18+ in Australia.