Well, GT6 began as a PS3 game and they keep supporting it for 2 more years ( last update is from december 2015 ). I like GT6, but the fact that the course creator was an online thing and it's not working anymore always made me sad :/
I still believe it will be a PS4 title: first of all GT never was a launch title.
GT HD was the only game that was going to be available soon after ps3 launch, but at the end the project was scraped and the first GT on PS3 was GT5 Prologue in 2007, 70cars per 6 tracks to begin with; for a full GT experience on PS3, we had to wait 2010, 3 years after console launch.
GTSport instead came out in 2017, 4 years after PS4 launch and 2 years after GT6 last update.
Plus, I believe PoDi wants to give their best on PS4 before fully create a superb PS5 GT Games. Think about games like GTA IV and GTA V : even if both are on the same console, it seems that they're from two different gen.