The DTM is very WIP atm.
The problem is, it's sooooo hard to edit GP4 that it'd take years to do! There would be other limitations too, like not being able to have individual cockpits or physics etc.
What do you mean the "graphics aren't what the could be in the game"? Can you not have the settings up on high, or do you think F1 2002's graphics aren't as good. I personally think that F1 2002's graphics, except shadows, are better than GP4! Anyway, IMHO
But a GT Mod for GP4 isn't really very likely.