Gran Turismo 5 took a while. It shouldn't surprise anyone that Polyphony Digital will require a smiliar amount of design time for the next installment.
According to European site Dinamica Games , Sony Worldwide Studios head man Shuhei Yoshida confirmed that Gran Turismo 6 is indeed in development, but it'll likely take "several years."
The follow-up assumption is that GT6 won't be ready until the PS4 is available. November will mark the PS3's fifth anniversary and everyone has been wondering when Sony will unveil the next piece of PlayStation hardware. However, the company has repeatedly said they have no plans to release the PS4 any time soon, so maybe GT6 will end up being one of the last great games for the PS3. On the other hand, wouldn't it make for an amazing PS4 launch title…?
Anyway, just remember that GT creator Kazunori Yamauchi is a perfectionist and if he's not fully satisfied with his creation, we won't see it. It's likely we won't see GT6 until late 2013 (just a guess), but what sort of hardware will we see at that time? The waning years of the PS3 or the infancy period of the PS4?