You aren't doing anything wrong, it's no bug, it's not a GSX problem, is exactly how's supposed to be, and
The vehicles you are seeing BEFORE you call GSX are those called "Airport Services". You think they are controlled by GSX, but that's just because they LOOK like GSX ones, since GSX not only adds custom Ground services to your airplane ( when you call it ), but it also acts as a replacer for default Ground Services, the manual clearly explains this at Page 6, where Airport Services are discussed. GSX has no effect on the placement of these vehicles, which are placed by the simulator itself according to the standard parking assignment rules, completely outside GSX control. They are just different models/textures, that's it.
After you call GSX for some service, the default Ground Vehicles are removed, and will be replaced by the ones controlled by GSX, using the GSX operator assignment rules, with their initial placement specified by the custom profile.
Basically, before you call GSX yourself from its own menu, what you see it's just default ground vehicles, with their models replaced by GSX.