Works perfectly fine here. Tried in both ways:
- by positioning on Gate 21 using the MSFS Main Menu, no message from GSX asking to reposition, selected Boarding, it worked normally.
- by positioning on Gate 21 using the MSFS Main Menu, selected "Reposition on this gate" in GSX, selected Boarding, it worked normally.
Yes, there's a difference between the MSFS-calculated position and the GSX custom Stop position that comes with the profile, but it's still well within the 25 meters threshold (which is default and the profile hasn't changed it) where GSX would still consider the airplane parked correctly.
Have you disabled GSX replacement jetway files for it, something you should always do when installing a new scenery after GSX ? Not doing this, would surely cause a parking position conflict.
And, when you use the PMDG, and your startup save states has the PMDG vehicles active, GSX will first send away all PMDG vehicles, which will take about 30 seconds, that might give you the impression GSX stopped, but in fact it's just waiting for the PMDG vehicles to go away, THEN it will start Boarding with its own vehicles. That's why we suggested several times on the forum to create a custom Save state in the PMDG to be used for GSX, with all PMDG vehicles already sent away, so you won't have to wait 30 seconds each time.