As explained in the manual, and of course the forum in in so many other threads:
GSX doesn't have ANY control over the jetway movement, it doesn't change or improve how they works and, if you are on a default airport where default jetways have been replaced by GSX, the only thing GSX has changed, is the jetway appearance, not the way it works.
What GSX does when you operate the jetways, is simply sending the default command toggle a jetway, exactly like as if you were calling the jetway from the default ATC Ground Services menu. If that jetway, using the same airplane parked in the same position doesn't work from the default ATC menu, calling from GSX won't change anything.
Because, the only thing that makes a jetway working or no, is its relationship with its own placement compared to the placement of the Main exit of the airplane you are using.
This is were using GFSX might change things a bit: it is possible that, following GSX guidance to park, might not place you exactly in the same position as if you were following MSFS own Marshallers, which might improve how jetways work in some cases, but not always. But this is only valid when there's no airport customization involved.
If you customize the STOP position in GSX ( or use a pre-made airport profile that has done that ), GSX will then try to place the Main exit of every airplane in the same position against the jetway so, it will be very likely that, if the jetway worked with the airplane which was used when creating the profile, it should work with most airplanes as well.
The chapter in the manual that explains this is at Page 44, and it's named "Understanding the Stop Position".
Another thing that might cause the jetway not working, is trying to use GSX jetway replacement files on a 3rd party airport. As explained on the manual at Page 7, the paragraph named "Jetway Replacements and 3rd party airports", GSX will automatically detect all 3rd party airports and exclude them from jetway replacement when you first install it, or when you Update it but, if you install a new scenery and don't update GSX, you must go into the GSX Config page, and click the "Exclude 3rd Party option" again to be sure the new scenery is being prevented to have its jetways replaced by GSX. And, if the scenery package is not named according to the official standard of developer-airport-ICAO-name, you need to add its code manually, because GSX won't recognize packages with non confirming names to be airports.
If you don't do this, and try to replace jetways in a 3rd party scenery anyway, they might even show up, but they won't work, because their association is to the parking of the default scenery there were designed for.
