I run FSX:SE via SIMstarterNG
That's likely your problem.
GSX must add its own SimobjectPaths configuration line to the FSX.CFG, that tells the sim where its objects are located, and is no different than any other product out there that use its own folder for Simobjects (like SODE, MyTraffic, UT2, etc.).
This is done at install time but, if you are using SIMstarterNG to switch between different FSX.CFG files, you must be sure ALL of them has been created AFTER installing GSX. Same for every other product out there that adds its own objects, they will all fail, if you use SIMstarterNG to switch to a different FSX.CFG file, one that was created before *they* were installed.
Surprised to know that a product which does something potentially very dangerous, like shifting between different FSX.CFG files, doesn't have a feature to recognize automatically changes to the SimobjectPaths lines made by installers without keeping them in sync with its other backup configurations.