GT6 will be a PS4 title, but probably not a launch title.
I've told Sony fanboys for half a decade already that PS4 will definitely be launching before 2016, but for years they clung to the notion of a 2016 PS4 launch based on comments about ten-year plans, not realizing that previous PlayStation generations had life cycles of around a decade even though they didn't go a decade between generation launches. A launch around the end of 2012 seems very plausible, and at the latest it might push 2013.
Whether PS4 launches in 2012 or 2013, GT6 will be doing good to make either of those dates. Considering GT4 was released in Japan in December of 2004 and North America in early 2005, and then it wasn't until the end of 2010 that we saw the release of GT5, it wouldn't be terribly realistic to expect GT6 before the end of 2012, as that would be one of the shortest GT development cycles to date. (GT to GT2 was two years, and GT2 to GT3 was less than two years, but GT3 to GT4 was four years and GT4 to GT5 was five years.) Even if they managed to get GT6 out the door by the end of 2013, a mere three years from GT5, that would still put it after a 2012 PS4 launch or around the time of a 2013 PS4 launch, but not before.
So, if they realize that GT6 won't be making it to PS3, and instead push for it as an early PS4 release, it will probably need more time for more overhauling of the engine and all, meaning it won't likely be there when PS4 launches, but may make it out within a year or two into PS4's life.
Incorrect. SNES ultimately outsold Genesis by quite a bit, but Genesis was out two years before SNES, and Wii launched a full year after the 360 and yet took the lead almost immediately and has held it every day since. And yes, Wii counts, because if we can simply dismiss the weakest platform of a generation then that means dismissing PS2 since it was weaker than both GCN and Xbox, and it means we can dismiss PSX (PS1) since it was weaker than N64.