Hi.
I had the same problem as OP as I deleted the AFCAD for Lake Hood (Z41). (Path FSX/Aerosoft/Anchorage/Scenery/AFX_Z41_sim-wings_04.bgl).
Upon re-entering FSX and trying to re-open GSX, I get a "System error while trying to access FSX/Aerosoft/Anchorage/Scenery/AFX_Z41_sim-wings_04.bgl. Cannot find file specified". GSX won't open.
I tried deleting the Z41 airport file in the AppData/Virtuali/GSX folder, still happening.
Any help greatly appreciated.
EDIT: Deleted ProgramData/Virtuali/Couatl and PD/MS/FSX/SceneryIndexes to let it rebuilt the cache. it has now reverted to using the default Z41 afcad (Scenery/0101/scenery/APXetc...)