It the gate has a jetway, there will be no stairs and no bus truck.
Of course, by "gate has a jetway", we mean a proper FSX jetway. If the jetway is "embedded" as part of the terminal, it will not be seen so, stairs and the bus will appear.
No program can obviously recognize a jetway, if the scenery developer hasn't separated it from the terminal model, and flagged as a proper jetway in the AFCAD, this is the main design choice in GSX: it will have to work without ANY modification to the scenery, which is the only way it can support every scenery from day one.
The downside of this, is that if you have airports with jetways embedded in the terminal ( all our airports are not like this, and all default airports always have jetways as separate objects ), it will be as if they will not exist.