The first thing to check is if GSX loaded the correct .BGL for the airport, in the airport customization page. Yes, you can open it even if you see that message, if you open it when you are NOT in a parking, for example on a runway.
This, assuming you have the SU10 Navdata API option Disabled, which is the default.
If you have the SU10 Navdata API option Enabled, GSX won't need to load or recognize the .BGL, so if you still see the message, the scenery might really have some problem, or there might be another scenery in conflict.