Rockstar has confirmed that Grand Theft Auto VI launches on PlayStation 5 and Xbox Series X and S. There is no last-generation version planned, which lets the team build for current hardware from the ground up.
A PC version has not been announced or dated. Based on Rockstar's history with GTA 5 and Red Dead Redemption 2, a PC release is widely expected to follow the console launch rather than arrive alongside it.
If you are a PC player, the realistic expectation is a wait of roughly a year after the console release, though nothing is official until Rockstar says so.