Does the installation of GSX place the FSX default airport vehicles so to speak in limbo, as some sort of inactive or backup file?
It only patches the default airport vehicles file to disable JUST the default tow truck, but any other vehicle in that file would be untouched and it will still work as before.
Before patching it, it creates a backup file named vehicles_airport.gsx ( in the Scenery\Global\Scenery folder ), so in case anything wrong happens, you can restore from it. If you don't have a file named vehicles_airport.bgl in that folder, it means that either the file wasn't there when you ran GSX, or something happen while patching the file, such as the PC crashing while patching it, which is very unlikely, but still theoretically possible.