This has been already discussed, if an airport has a smaller airport with a different ICAO inside it (like a military base or a seaplane base), GSX might assume you are on the smaller airport.
To fix this, you can disable GSX for the small airport ICAO, by editing the %APPDATA%\Virtuali\Couatladdons.INI file, the parameter
disable_on_airports = ( put here a list of disabled airports, separated by spaces )
Also explained on Page 19 of the GSX manual.
