The problem doesn't seem to be specific to GSX, as it doesn't work neither from Fenix EFB.
If doesn't work from the Fenix, it's a problem in the sim itself. Which might possibly be introduced by GSX, but only if you forgot to Exclude the GSX Jetway replacement files for that airport, if it's not default.
Failing to do so, will result in the GSX Jetway replacement file adding conflicting data to the sim ( usually noticeable as double parking spots in the World Map view ), which makes the jetways not working, because the GSX jetway replacement files are associating jetways on a parking name/number based on default sceneries, that's why they must be Disabled at 3rd party airports.
Of course, that's the only one of the possible reasons, but if you are sure you Disabled that airport from GSX Jetway replacement, it might be a similar conflict, created by another scenery in conflict. For example, at both Paris airport (LFPO and LFPG), the Asobo own Voloport scenery cause a problem with Jetways not working, if it's not removed.
