Even though GTA is a huge IP, and game-to-screen adaptations are on the upswing, I don't think it would translate to film very easily and R* is smart to steer clear. Gangster movies are a very crowded field, and it's hard to make one that stands out when you're up against the likes of Heat, GoodFellas, and The Godfather.
Also, like a lot of "cinematic" video games, GTA plots tend to be low-key derivative of classic films and they won't feel as fresh when you turn them back into screenplays. They're appealing because they feel like a living version of a Scorsese, Brian De Palma, or John Singleton crime drama - take away the interactivity, though, and it's just another movie.
@asmith announced as in production by Rockstar/director/producer/etc, or a release date, or an official trailer