A few years ago there was a very reliable-seeming leak from a source close to Rockstar that said that the game was happening. Shortly after, Rockstar made the source clam up (maybe paid him off) and since then all we've heard is how the game is not happening.
I think there's a good chance the original leak was correct and we've been getting smoke screens ever since. Rockstar doesn't like leaks.
Edit: Or maybe this is a remaster/remake.