I think you need some clarifications here:
- The only sure way to know if the jetway is made by GSX or it's coming from an external SODE file, is to check the GSX scenery customization page. If the jetway is provided by SODE, it will labeled with "Parking has an XML Jetway"
- Regardless how the jetway is made, this is completely separate from the GSX choice of ground handlers. When you start a service, GSX will always give you a choice of a ground handler ( unless only one is assigned to that parking/airport ), but this is unrelated to what logo will be displayed on the jetway, the jetway logo in GSX is assigned following a different ruleset than the ground handler, and so the catering trucks, they all have their separate scoring rules.
This, assuming the jetway *IS* made with GSX. Not that this would change in any way how GSX selects an handler, it would be the same even if there was no jetway at all.