This is what it looks like if I ask AES to position the a/c in the correct location but then as you can see GSX will no longer realize I'm parked at a gate.
GSX checks your airplane position against the parking center, in order to be considered parked, the distance of the airplane reference point from the parking center must be less than 15 meters, so you were probably more distant than that, maybe AES is simply more tolerant.