You don't have a problem with GSX.
As explained so many times on the forum, and of course on the manual, Page 5, when jetways are explained ( in your case, the scenery doesn't even have GSX jetways, but the principle is the same ), GSX doesn't have ANY control over how the jetway will animate, the "Operate Jetway" simply send the standard Jetway toggle command to the sim, exactly as if you called the jetway from the default ATC ground services menu.
The one and only thing that makes a jetway working or not, is your parking position in relationship to the jetway placement so, perhaps you have being mislead assuming GSX is a problem (because you see jetway working on AI), but that's just because those AI might have been parked differently, enough for the jetway to work.
What you CAN do in GSX, which is also something that usually solved by GSX airport profile creators, is to adjust the custom Stop position in the GSX airport profile, enough to have the Main exit placed in a way that would help the jetway working better.
I suggest reading the manual, Page 44-45, the chapter named "Understanding the Stop position", which will make you understand the issue, and how you can use GSX customization ( or just download a pre-made profile for that airport, if it exists ) to help jetways working better.
