First, be sure GSX is reading the correct file for the airport. The GSX parking customization page will tell you the name of the AFCAD that is being used by GSX.
Assuming it is, GSX cannot operate if the parking is isolated, meaning it's not connected to anything because, if it's not connected to anything, it would be impossible for GSX to find a path for its vehicle to reach the parking ( and of course, AI won't use it too ).
You can try opening the scenery AFCAD and confirm with ADE if the parking is really unconnected ( either visually or using the "Fault Finder" tool ) and if it is, I suggest reporting the problem to the scenery developer.