All these service vehicles should be pulled back to some safe and close proximity to the terminal building.
Which is why, GSX being the flexible program that it is, allows you to customize the vehicle starting positions for each parking, which is the same position they'll return to after finishing the service.
This cannot be calculated automatically everywhere, so the default position is somewhat related to the radius of the parking spot, which is the only thing GSX can "see" of the outside world, together with other data in the AFCAD.
That's why GSX can be customized a lot, in every parking of every airport, so you can be able to fit it perfectly to your own favorite airports, and we won't have to charge you per-airport, should we tried to do such customization ourselves. You'll find that most of the FSDT sceneries already come with customized vehicles positions, so they don't usually require much touch-up.