Which platforms are confirmed?
PlayStation 5 and Xbox Series X|S.
Those are the only platforms named on Rockstar's product and support pages.
The evidence
Explicitly stated by an identified primary source.
Rockstar GamesGTA VI official release and platform listingRead the full supporting guideWhat would change this answer?
A new official platform listing.
What changed
Official product listing rechecked September 23; the answer is unchanged.