This has been discussed many times on the forum: there's nothing to fix in GSX, which simply reads the AFCAD parking radius and compares with your airplane wingspan. If the radius is smaller than half of your wingspan, the parking is considered too small. This is exactly the same standard FSX applies for AI airplanes.
GSX can't obviously know if the parking is really small and doesn't grant enough space around (so you are really not supposed to go there with that airplane), or is too small because of a scenery designer mistake so, it doesn't have any other choice than to trust the AFCAD.
This means, to "fix" this, you need to check the scenery AFCAD, and eventually correct it.