I’m posting here to clarify whether this behavior can be influenced or controlled by GSX, or if it is indeed entirely handled by the simulator itself.
If you see something appearing as soon as you arrive on an airplane, you can be SURE it's not GSX. They can be either:
1) default vehicles spawned by the simulator itself
2) ground vehicles supplied with the airplane
3) static ground vehicles supplied with the airport.
GSX will surely remove #1 and will try removing #2 (some 3rd party planes don't allow to remove their own ground vehicles), but it can't do anything to remove #3.
And, in any case, GSX will only try to remove objects around you, and only after you call it. Under NO circumstances GSX will automatically create ground vehicles "just" because you arrived at a parking spot, it will always require calling GSX first, so it will first try to remove the existing objects and then it will try to create its own.