Please clarify, exactly if the installed airport is default only, of you have an add-on as well.
I tried with the default only, and I see all parking spots are a bit small, so I get the "Parking is too small for your aircraft" message when opening the customization page with something like an A320, which in fact should be allowed to operate there, at least on some parking spot. It should be enough to set the Max Wingspan, by selecting all gates you want to fix, and click the small airplane icon next to the Max Wingspan parameter.
However, there's seems to be another problem with the default scenery, and it's all parking spots on the main Apron are rotated 90 degreess right, not matching the ground markings which are oriented normally. Checking the airport on Google maps which shows some planes parked, all in the expected position, aligned with the markings, so clearly the default airport has issues, however, GSX should still be able to handle it, since other than the weird orientation, everything can be customized normally and should work.
However, could be an add-on tried to FIX the above problems, but it adds its own issues as well ? Note that, when you use the Navdata option, GSX is taking whatever you get in the sim INCLUDING CONFLICTS. When GSX loads a .BGL, we only got data from a single file so, either you load the correct one or you don't, but when the Navdata option is Enabled, GSX will get everything, including possible conflicts between two files for the same airport. since it no longer knows where the data is coming from.
