Although Gran Turismo 6 has yet to be officially announced, a Nordic retailer has already listed the game on its website saying it will be on the PS4.
The same website also lists the game being available on the PS3 as well. This could suggest that Gran Turismo 6 will be a cross-platform release.
Sony is to hold a special announcement regarding the release of Gran Turismo 6 on May 15th. Rumors suggest the game will be out for the PS3 later this year. This further evidence suggests that a PS4 release might be likely too.
We should find out more official information on the game later this week so stay tuned.