I'll clarify again, because my reply might not be very clear:
- On arrival, you just CAN'T use the airstair, unless you don't select ANY gate, park on it, and select it only *after* you shut down the engines and set brakes. This is a known GSX limitation we need to work on, likely with a new option in the menu that will ask you in advance if you are planning to use Airstairs, when you select a gate on Arrival.
- On departure, you must extend the staircases before doing *anything* with GSX, that is before GSX created any objects, because when its objects are created is the time when, depending on the airstairs status, that GSX decides to create or not the parking front stairs.