GSX use the scenery AFCAD information to decide where to spawn support vehicles, using a priority system:
- Fuel vehicles will spawn from parking of the "Fuel" kind. If there aren't any, they'll spawn from a parking of the "Vehicle" kind. If there aren't any, they'll spawn from a random empty parking
- Catering vehicles will spawn from parking of the "Vehicle" kind. If there aren't any, they'll spawn from a random empty parking.
So, if you think GSX is spawning vehicles from the "wrong" place, it means the airport you are using hasn't been designed correctly, lacking parking of the proper type in the proper positions. This nothing GSX can fix by itself, of course, but it must be sorted out by the scenery designer.