As explained in so many other threads on the forum, whenever you have a problem of mismatched positions, the problem is always caused by a conflicting scenery coming from another product. The most common case of conflicting sceneries are those coming from AI Traffic products, that must be removed if you have an addon airport.
You can check the name of the AFCAD used by GSX with the "Parking Customization" menu, and if it's not the one that comes with the scenery, you must exit the sim, remove or rename the file, then restart the sim, and choose "Restart and rebuild the airport cache" option from the Couatl menu (if the change is not detected automatically).