GSX replaces all default Ground Vehicles, which are the ones that both serve AI planes ( Airport Services ) and also wanders around the airport ( Living World ), using its own models.
They will behave like default ground vehicles, so they won't do all the same things GSX can do for your own airplane, but they surely improve the look of the airport with hundreds of different region-specific operators and better looking models in general so yes, even if you don't use airliners, GSX can surely improve the appearance of airports.
