it's just that GSX "thinks" all of the gates are too small.
GSX doesn't "think" anything. It just READS and BELIEVE what's inside the AFCAD, because it cannot possibly decides by itself if a spot is large enough to hold a certain kind of airplane, so it can only rely on the information in the scenery, which is the parking radius in this case, so it simply compare it with the airplane wingspan divided by 2, and if it's less than that, it will say the parking is too small for your airplane.
So, either the parking radius IS too small, or the wingspan in your airplane.cfg is too large.
Now, there might be many reasons why the developer set a smaller radius than what it might physically fit that place, or it might just be an error, but surely GSX cannot decide on its own.
But of course, you can override this with the maximum wingspan setting, as explained in the manual.