I was under the impression that the GSX.CFG file would have priority over the Aircraft.CFG as it has worked on my other A/C.
You still don't understand, see the above reply from bollerbill:
The jetways are completely controlled by FSX. Other than providing the "Remember to operate the jetway" message, GSX has nothing to do with their operation.
This is also clearly explained in the GSX manual: EVERYTHING related to jetways operation is entirely unrelated to GSX, the ONLY thing that GSX does, is checking if your parking has a jetway or not, so it will display front stairs (or not) and will remind you (or not) to operate it.
But, as explained many times already, jetways themselves will continue to operate (or not) as they were without GSX. The GSX.CFG file HAS priority over the aircraft.cfg, but for GSX use only, to control GSX vehicles/stairs placement. FSX doesn't know anything about it, and it's FSX that operates the jetways, not GSX!