Maybe a search of the forum, or maybe a read of the manual ?
As explained many times, GSX reads your AFCAD. If the radius of the parking specified in the scenery is smaller than the airplane half-wingspan, it will say the parking is too small. GSX can't obviously know if the parking is REALLY too small, or it's a mistake of the scenery designer, or it's your mistake, which are trying to park in the wrong place.
How to do this, is explained in the GSX manual, look the page when the Airport customization is discussed.
Of course, I assume you use the most current GSX version, with all the last updates. If not sure, download it again and install it.
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From Virtuali (Admin) http://www.fsdreamteam.com/forum/index.php?topic=6855.0