Another reason why 3rd party airports are not detected, is because they failed to Exclude things like jetways or taxiways from the default airport, so GSX assumes they are not complete airports, but just enhancements to the default airport, so it will use the default airport instead. I'm fairly sure this is the problem with Gaya LOWW, but there might be others made like this.
However, if you enable the SU10 Navdata in the GSX Config panel in the installer, it won't use the airport .bgl anymore, and will ask data about the airport directly to the sim itself, using Simconnect.