ok Termin8Anikin X = 10 in roman Numerals...
I don't mean to sound mean or anything just I wanted
to correct that. V = 5.
so No F-Zero X was not the tenth in the series...

but...
It was just a letter to distinguish it
without having to add a number to the end
just something that sounds cool like
Z-28
also...Wasn't there a 64DD version of F-Zero,
an addon pack with a track editor or something?
anyways...that has nothing to do with it this now

Anyways I think GX is just a set of letters to make it sound cool