The parking sizes in the scenery are correct, taking into account several FSX limitations, for example moving jetways (parkings too large that overlap will activate the wrong jetway) and to sort AI planes correctly.
A real airport doesn't have *that* many parkings that can suit a 777 and, if you use too dense AI settings, it's possible there will be none left available.
As explained on the GSX manual, in the "Airport Customization" page, you can override the parking radius used by GSX, so it won't affect the rest of the simulation, but will allow the parking to be used by GSX.