Well - are you simply talking about percentages here - you need to get golds in ALL the GT mode events - all races and complete series (not the licenses though, you need at least bronze in those), complete the Arcade mode at the hard level, and get golds in all the Time Trials in Arcade mode. This should give you 100% (providing I haven't forgotten anything!) in the game status screen.
From there - well, you could set yourself a goal of getting all the cars, winning the F1 series in a non-F1 car - there's plenty to do.
Oh yeah - moving this to the GT3 forums...