While it is too early to talk about the next Gran Turismo, Kazunori Yamauchi told Complex that Polyphony Digital started working on the 6th installment of the series. Don't worry, based on how long they took to develop the 5th game, we're talking about years from now.
If you ask me , Gran Turismo 5 might be the only GT we'll have on the PS3 as I see it has many years left in it. They're still busy supporting GT5 at the same time just placed a huge update on the game. It's constantly evolving at the same time taking so much space in your PS3. It took Polyphony Digital 5 years to get released since it's announcement back in 2005.
Is it time to talk about the 6th installment? too early or not, Polyphony Digital is starting work early.