The problem doesn't have anything to do with GSX.
It's the scenery that must have a "hole" in the terrain there. It might be the scenery itself, but it might be also a 3rd party mesh, which the scenery you are using isn't covering properly with a flatten file, an AFCAD apron or a platform.
You can TRY setting the Ground Elevation is GSX, by clicking the small airplane icon on the top right of the scenery customization window, but it might not work with every scenery.