I pose to you all this question:
What does the 'X' in F-Zero X stand for?
I think that if we never know the answer to this question, so will it be with F-Zero GX.
It can't be '5', since F-Zero X is the second game in the series, and F-Zero GX is the fourth.
So what can it possibly mean? Only E3 can tell. If not, then the answer will remain in the bowels of Sega and Nintendo.