This usually indicates a problem with the scenery AFCAD. Try to force a complete rebuild of the GSX scenery cache, by removing the following folder:
C:\ProgramData\Microsoft\FSX\SceneryIndexes
(note that C:\ProgramData is hidden, so you'll have to enable hidden folders view in Explorer)
This will force both FSX and GSX to rebuild their caches. When GSX it's done, exit from FSX and enter it again, and see if the problem gets fixed.
