I have a problem with Jetways not quite reaching the aircraft and stopping short so consequently passengers walk in mid air for a short period till they reach the aircraft. This even happend on my payware airports, and before you ask, yes i've excluided the payware airport jetways in the configurator.
Since it happens with 3rd party jetways too ( it happens with default airports with default jetways ), it indicates GSX cannot possibly have anything to do with GSX, and it's a know limitation of the default Jetways animations system, which can happen even with default jetways on default airport. Fact you call a jetway from the GSX menu doesn't change how the jetway would work, since the only thing GSX does to call a jetway, is just sending the standard Jetway Trigger command, without any involvement how the jetway works ( or not ).
While it's entirely normal that a jetway might not work on a certain combination of airplane, jetway model and placement and parking position, the real problem is there's no way for GSX to KNOW where a jetway docked, or if it has docked partially, nothing in the SDK allows that, so GSX can only assume it has docked correctly, and will generate passengers based on that assumption so, in a way, when you see passengers "walking in the air", passengers are the only thing working in that scene, because the real issue is the jetway docked incorrectly, with no way for GSX to know that.
The only thing you can do, from GSX, is customize the STOP position, to ensure the airplane will end up in a place where the jetway would work and, since the Stop position is relative to the airplane Preferred Exit ( different planes will stop with their own Preferred Exit in the same position ), you can "reasonably" sure if it works for an airplane, it should work with many others. That's why there's a Jetway TEST function in the Parking editor ( NumPad5 while editing the Parking Position ).