It's a game where you go around and do missions, and shoot people. It's fun as hell. I can't believe you don't know what GTA is.
There's a whole series too. There's GTA III, GTA Vice City, GTA San Andreas, GTA IV, and GTA V. GTA V is the one people always talk about.
Even I've heard of GTA! I've never played any of the games though. That type of violence is not for me.
I've enabled Star KOs in all stages in Smash Ultimate as well as upped the frequency of them. Seeing characters get Star KO'd in the Wii Fit stage is hilarious.