A common reason why an 3rd airport hasn't been recognized by GSX, is the airport file lacking a proper DeleteAirport command, which is normally supposed to be present, when a airport replaces a default one.
This might be intentional, for example the 3rd party airport might just be an enhancement of buildings and texture but might still use the exact same parking names/positions and the same taxiway routes from default, so in this case, lacking an DeleteAirport command to remove these elements is correct, but GSX simply doesn't support this: when a .BGL is used, it's assumed to be self-contained, if a scenery requires the underlying default airport, GSX won't load two .BGLs at once, so it will only use the default one, which in THIS case would be correct, since that airport needed the default data anyway.
However, most of the times the 3rd party airport DOES change parking and taxiways, and the lack of a DeleteAirport command to remove the default one is just a mistake, because it would cause conflicting parking/taxiway data. In this case, the airport developer should fix it.
However, ALL of this is only valid when the SU10 Navdata is NOT used, when GSX reads .BGLs. If you enabled it, GSX won't need to read the .BGL anymore, so it will take whatever airport data is present in the sim, through the Simconnect Navdata API.
Note that, it will take everything, conflicts included...