It doesn't have to stand for anything, and the only reason I don't think it's a typo is that there have been other errors on the e3 pages (bad links, wrong pictures, etc.) and they've gone back and fixed them within 24 hours.
I'm not saying it IS "GX," just that assuming it's a typo isn't the best idea -- everything they put up is proofread and checked, and they get tons of emails regarding changes on their website. F-Zero has never followed the naming convention of adding the name of the console to the game title (there was no "super f-zero" nor "F-zero 64"), so using GX would both show that it's the "next game" without saying "on gamecube!" necessarily.