According to Impress Watch (translated by Andriasang), Yamauchi said that he’s been working on the framework for Gran Turismo 6 immediately after work on GT5 was finished.
He also added, “We’re always aiming for the latest technology and the latest expression;” which heavily suggest that GT6 is will be released on the PS4.
In addition to this, Yamauchi was also asked about the possibility of a Gran Turismo game on the PlayStation Vita. While he didn’t say anything concrete to confirm or deny that they’re working on it, he shared that if he were to make it, he’d make use of the handheld’s ability to play anywhere and its 3G capabilities.
I don’t doubt that we’ll see a new Gran Turismo game again, but let’s just hope it doesn’t take as long to make this time.