As explained many times already (in the manual, in the forum, etc.), GSX doesn't change or control anything related to jetways, they will continue to operate exactly as they were before installing GSX and the GSX installation doesn't change a bit about any jetways behavior in a certain airport/airplane.
In this case, if the scenery developer has configured only the high bridge to be animated and the airplane developer has configured the main door to the the 2nd on the left, the first door won't get any jetway, since FSX supports only one animated jetway for each gate, and it will connect to whatever door is flagged as being the main one in the aircraft.cfg
Using/installing GSX or not, won't make any difference, and there's nothing in GSX you can set to change this, because there's nothing in GSX that affects jetways in the slightest way.