This is what I did to get my GSX installed and working again.
I finally got my GSX back up and running.
I have Win10 with MFSX-SE.
First, I opened my Control Panel, Win10, Programs and Features.
Then I uninstalled everything with FSDT programs, including Addon Manager when prompted.
Then I rebooted my computer.
Then went to FSDT website and downloaded GSX.(saved to external harddrive.)
Then I installed GSX to my computer, but did not, repeat did not run the updater.
Then I opened my MSFX and let it load up.
MFSX opened with GSX fully operational and working.
Maybe I just got lucky, but you can give it a try and see if it works for you.