GSX reads the radius of the parking specified in the AFCAD or the airport and, if it's smaller than your airplane wingspan divided by 2, will say the parking is too small, which in fact is, provided the AFCAD was designed correctly.
So, either the AFCAD you use has been designed wrong, with a too small radius compared to the actual clearance of the parking spot, so you should fix the parking in the AFCAD or, if the AFCAD is correct, you shouldn't park there in the first place, since the airplane wouldn't fit.
These are exactly the same rules that FSX itself uses both when assigning a parking with its own ATC, and when assigning a parking to an AI.