I speak to a few industry veterans on a regular basis due to work, and they're convinced GTA 6 is 2025 at the earliest. Even then, it might not even feel like the kind of GTA that many want it to be. It'll likely be less focused on story, and more of an online evolving map for multiplayer purposes. GTA as we once knew it is dead now IMO. It's gotten too big. The highest grossing media franchise of all time. You don't rip up that business model and start again - the suits won't allow it.
Maybe if we're lucky we'll get modern remakes of the earlier games or something.