It means the program has detected you changed something in the Scenery Library, just like the similar message in FSX. It shouldn't appear if you just load an airport, but changing layer order in the Scenery Library will trigger it too, just like the FSX default scenery cache message.
Fact you have GSX or not is not really relevant: the ability to read the active .BGL it's a service that is offered by the Couatl program to any plugin that asks for it, right now only GSX requires it, but other products might use it, even sceneries, but not only.
