Your problem doesn't have anything to do with GSX, which is further proved by the fact you even have partial buildings (GSX won't affect buildings in any way)
Check you are not running FSX into XP Compatibility mode, and be sure you have turned off Transparency Antialiasing ( or Adaptive Antialiasing, it's the same thing ) in the video card control panel.
Also, try to update your video drivers, and also reset the video drivers control panel to all default settings.