The issue with the 757 cargo works but only if I select the cargo ramp by pressing ctrl F12 and making a selection. if I just use FSX to place the aircraft at a random cargo ramp, I get the same message of aircraft not parked correctly.
Well, of course it's like that!
FSX doesn't know anything of the carefully programmed custom stopping position of the scenery which are set in the GSX customization file that comes with the scenery. It will just place the airplane in the middle of the parking...
When you use the GSX "Warp me" feature, it will immediately move the plane in the same position you were supposed to park had you taxied manually and followed the Mirror docking system indications correctly so, it will work in both cases, and this position will result in the airplane Main Cargo door always aligned to the loading platform, regardless of the plane used.
Provided, of course, the Main Cargo door location has been set correctly in the GSX configuration.