The problem doesn't have anything to do with GSX, it's just an issue of your airplane contact points not being correctly specified in relationship to the the static_cg_height parameter and the strut compressions.
GSX doesn't do anything strange, other than just setting the airplane in Slew mode, so the door customization won't be affected by the airplane load, so only the static value will be used, resulting in the door information to be saved in the same way, regardless of the airplane load at that time.
For the same reason, what you are looking at is exactly the same effect as if you enabled the Slew mode yourself, which is simply what GSX does while in the editor.
And, it's not an issue, since even if the airplane would look wrong in Slew mode, the door position is saved correctly, and it's relative to the airplane reference point so, when the airplane will be used normally, the vehicles will be placed correctly, with different airplane loads.
