Two things we know: Kazunori Yamauchi is one of those rare visionary perfectionists, and as a direct result, new Gran Turismo installments take some time. Lots of time.
As reported by Japanese source Impress Watch (and cited by Andriasang ), the Polyphony Digital boss fielded a few ultra-burning questions concerning upcoming GT iterations.
Regarding GT6, Yamauchi confirmed that he's been "working on the framework" for the new effort ever since GT5 was out the door. Obviously, he's not about to say when we can expect it (at this rate, it might be safe to assume it'll be a PS4 title), but he did add something very much expected: "We're always aiming for the latest technology and the latest expression." Well, duh.
Then Yamauchi said a PlayStation Vita Gran Turismo was possible but if they were to make it, they'd want to utilize that "play anywhere" feature via 3G functionality. It's likely he'd want it to connect with the new PS3 – or PS4 – Gran Turismo as well; we keep hoping more developers will take advantage of that "transfarring" feature pioneered by Hideo Kojima.
Just let us know when GT6 is coming. We simulation fans will be ready and waiting. And drooling.