You know ... it depends on a couple of factors:
1. The scope of GT7
2. When they started.
You could take GT6, recompile it for full 1080p, add some tracks and some new cars and release it for the PS4 ... and I'd buy it. Why? Because it's a successful franchise providing the best online racing experience that I've experienced ... and I'd pay to have the latest eye candy.
Here's another thing ... don't think that Sony doesn't have any influence here. They just released a new platform. It's in their best interest to see that all the major titles migrate over because they know the faithful will follow. There could be financial incentives behind the scenes to release soon.
Sooo ... it depends.