Found a fix that got my GSX activated on windows 8.1 64 bit.
Firstly make sure that FSX is De-Activated in the addon manager. If you can't deactivate it then click the reinstall button and enter your serial key when promted. Try deactivating again should work.
After deactivated restart flight simulator x and start a new flight. Go into the addon manager and activate GSX, once completed exit the addon manager. It will still show the demo airports in the GSX menu (that's ok this is normal!) now in the add ons tab in the top menu bar and in the GSX tab, click "restart coutal"
After about 15 seconds Coutal should have restarted and your GSX menu should now allow you to warp to a parking space and request ground services.
Worked for me after hours of fiddling today!
Also I have restarted fsx and my computer many times and GSX functions properly without having to repeat this method.
Hope this helps!
Any questions I would be happy to answer!