In what looks to be a pro-consumer move, purported Microsoft guidelines prevent publishers from selling "next-gen upgrades" as paid DLC. This means that Xbox Series X owners might not have to double-dip on cross-gen titles to get enhanced visuals and features.
https://www.notebookcheck.net/Rumoured-Xbox-Series-X-guidelines-ban-publishers-from-selling-next-gen-upgrades-as-paid-DLC.480651.0.html
If you must keep track of generations like that, GTA V is actually from the 7th generation consoles (PS3 and Xbox360), the 8th generation title is an enhanced remaster.