I just ended my Teamviewer session with Gary and, as expected, the problem was not caused by GSX or was a GSX bug. The issue was the known problem of the FSX.CFG file missing several default paths for Simobjects, that are always supposed to be there, otherwise hundreds of objects in the default scenery will miss, not just GSX.
The missing SimobjectPaths lines are either caused by having edited the FSX.CFG file by hand, or by some defective installer of another product we still haven't been able to identify.
In any case, these are the lines that are supposed to be present on a freshly installed FSX in the %APPDATA%\Microsoft\FSX\FSX.CFG file:
SimObjectPaths.0=SimObjects\Airplanes
SimObjectPaths.1=SimObjects\Rotorcraft
SimObjectPaths.2=SimObjects\GroundVehicles
SimObjectPaths.3=SimObjects\Boats
SimObjectPaths.4=SimObjects\Animals
SimObjectPaths.5=SimObjects\Misc
If you are missing any of these lines (especially the Misc folder), no FSDT product will work, and you'll also miss hundreds of default objects. If you have other products that comes with their own paths, like SODE or other products, you'll see their lines too, so be sure they are all numbered in sequence, with no "holes", like this example, with SODE and Aerosoft Split:
SimObjectPaths.0=SimObjects\Airplanes
SimObjectPaths.1=SimObjects\Rotorcraft
SimObjectPaths.2=SimObjects\GroundVehicles
SimObjectPaths.3=SimObjects\Boats
SimObjectPaths.4=SimObjects\Animals
SimObjectPaths.5=SimObjects\Misc
SimObjectPaths.6=Aerosoft\Split X\simobjects
SimObjectPaths.7=C:\Program Files (x86)\12bPilot\SODE\data\SimObjects
SimObjectPaths.8=C:\ProgramData\12bPilot\SODE\SimObjects
