You must have disabled this airport in the past and forgot about it. Our installers, of course, will never remove anything you made yourself, in order not to trash your own work.
The instructions to disable an airport are found at page 24 of the GSX manual, and they say you can disable an airport by adding its ICAO code to the disable_on_airports parameter of the following file:
%APPDATA%\Virtuali\Couatladdons.ini
By default, on a clean GSX installation on a brand new system, the line reads like this:
disable_on_airports = NONE