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 Games SupportGrand Theft Auto VI platforms, editions, and versionsRockstar GamesGrand Theft Auto VI official siteRead the full supporting guideWhat would change this answer?
A new official platform listing.