well gt HD is just that a NAME .
They could have called it GT superduper 2000
Its the new buzzword since "32" and "64" bit and "SUPER" etc.. from back in the days
Super mario, super galaxy, mario 64, 32 bit bla bla super turrican.
Now , its Stardust HD, GT HD etc. they use the word to make it sound nextgennish
all that PLUS what pjbliverpool was talking about a bit