GSX will try to put your airplane with its center point ( the red cross you can see in the top view ) in the center of the parking radius, as defined by the scenery AFCAD. This would be the SAME position as if you moved to the airport using the standard FSX "Go To Airport" menu.
So, if you get parked in the wrong place using the default "Go To Airport" feature in FSX, you cannot expect GSX to do any different, since that position is what the scenery AFCAD has set for that parking.
So, you must then check this first.
If, instead, you get a good position in FSX, and a different one in GSX, it's possible that, for some reason, GSX hasn't used the same AFCAD as FSX. You can check this using the "Customize parking positions" option in GSX, that will tell you the name of the AFCAD used by GSX, so you can verify if it's the correct one.
