To remove the default pushback trucks the GSX installer patches the default Scenery\Global\Scenery\vehicles_airports.bgl file to disable the trucks. The original is renamed to vehicles_airports.gsx. You can restore the original file by copying it over the vehicles_airports.bgl and it will revert to default. Be sure to make a backup of the patched file in case you want to disable the default trucks again. Be aware though, the GSX pushback will most likely run through the default one since the GSX vehicles don't recognize the existence of other objects.