Please clarify, is this a scenery that originally came with SODE XML jetways, but you removed them to replace with GSX ones ?
If yes, which model you used, precisely ? Fact it goes to the rear door, seems to indicate you choose a model that is too long, even on the minimum extension, to reach the front door. Or, you parked too far. Or, a combination of the two.
In the GSX editor, when you are customizing a jetway, pressing the NumPad 5 key, will "test" it using the current airplane in its current position, and if you hear a "ding" sound, it means it will work on at least one door, with an indication which doors will work.
Using this function, you will know if the jetway selected is right for the parking. Or, if the Stop Position is set correctly for this jetway. Or a combination of the two.