GSX doesn't support sloped airports.
If at least the apron is flat, we could add a customization file for it, setting a custom altitude which might help in case of sceneries made using non-standard elevation methods (like Aerosoft Madeira or La Palma), but it still must be flat (at least the apron).
In order to make a custom elevation file, I would need to know the following:
- The name of the AFCAD used by the scenery
- The precise, exact, elevation at the apron level.