GSX will remove all default ground vehicles from the parking, when you select it for service. If it doesn't, this seems to be indicate you have a conflicting AFCAD, so the reference position from which ground vehicles (50 mt. radius around the center of the parking in the AFCAD) will be removed, is not the correct one.
You can check the AFCAD used by GSX in the parking customization dialog, to be sure the one in use is the one that comes with the airport.