Please clarify what do you mean with "GSX isn't running". Are you missing the GSX/Couatl menu from the "Addons" menu bar, or you just call GSX with the keys, and it doesn't show the transparent in-game menu ?
If you are missing the GSX and the Couatl menu at start, the most common cause is that you started the sim too quickly after closing the previous session, not giving enough time to close all the opened applications and release all files, which might take some time, if your system is overloaded by addons, each one requiring to be properly close. If this happens, the next session won't be able to start, because there's still the old one stuck from the old session.
You can check on the Task Manager if there's still a Couatl.exe running from the previous version after the sim closed and, in case, Terminate it manually.