Many of our airports requires GSX, to create Docking systems, Jetways, etc.
Since all these graphic elements (textures, models ) are shared and
use the GSX code to work, you MUST install GSX to use the airport.
I did install it a while ago but all my default jetways were floating in the air but when I uninstalled GSX everything was fine until this recent KMEM reinstallation.
That wasn't caused by GSX, of course, but by OrbX vector, that change the elevation of all airports to conform them to their mesh. It would have been enough to disable the airport in OrbX AEC utility.