Gran Turismo HD was/is a prototype used at E3 to showcase the technical capabilities of PlayStation 3 - like, the GT HD "special playable demo", as Gran Turismo creator Kazunori Yamauchi termed it during the videogames jamboree, running in 1920 x 1080p at 60 frames per second.
Yamauchi also said such giddy things as, "Comparatively, the visual quality in Gran Turismo HD is 12 times higher than Gran Turismo 4 on PlayStation 2, allowing for a world of true high definition, not possible in any other visual format today."
So, we're not speculating too wildly about the 'real' next-gen Gran Turismo based on what we've seen of GT HD, but it may not be too long before Sony lifts its skirt to give us the full monty. Yamauchi said that GT next-gen may follow PS3's launch in November sooner than we think - hopefully the big announcement's coming shortly, then.
http://www.computerandvideogames.com....php?id=140869
All I can say is... OOFT!