I had the same issue when loading a new flight where I picked the same stand. Where I assume the aircraft will be positioned correctly.
You can't assume the airplane is parked correctly ( with "correctly", meaning a position that would result in the jetway working ), when starting a flight from the Main Menu because, when you start a flight from the Main Menu, the airplane is positioned by MSFS and GSX has no effect on this.
Your position will only be based on what's in the airport scenery file so, if the airplane used by the scenery developers is different than the one you are using, it might not be sure the jetway they might have tested to be working ( assuming they have tested it... ) will work on another airplane.
GSX might use a different positioning, because it tries to place any airplane with its own Main Exit on the same place ( resulting in different positions of the main gear on ground, like in real life ), so the GSX automatic calculation might work better. And, if you have a custom profile for that airport, hopefully whoever made the profile, has specified a custom Stop position too, which will work even better for jetways assuming, of course, whoever made the profile set it correctly and tested the jetway to be working so, the way GSX works, regardless which airplane was used to create the profile, it should work with almost every airplane, since GSX will place the Exit on the same predictable position, which will help solving the jetway.
This of course is valid even if you don't have a custom profile, so you can just set a custom Stop Position yourself, and use the test function in the Parking Editor ( NumPad 5 when editing a Parking ), to help you finding a good position that will work on that particular stand with that particular jetway.
That's why GSX has a "Reposition airplane", with the first option being "Reposition here". It's used to place the airplane in the "GSX position", instead of the "default position", since they are not usually the same.
All of this is of course explained in the GSX manual, the chapter named "
Understanding the Stop Position" at Pages 44-45