Okay, so time for a technical post. What's the engine behind GTA 6?

Okay so obviously it's the RAGE engine but how does it differ from GTA 5's rage? For example id Software use different version of their id Tech engine, with Doom: The Dark Ages being built on id Tech 8. Is R*'s engine the same way or what?

Now that I think about it, how did the RAGE engine used for RDR2 differ from GTA 5?