You can’t reinstall GSX Pro in “Offline” mode. You must always install it with the normal updater online, and then and only if you have reasons to suspect the updater can’t download the latest files, install the Offline installer after you do a normal installation.
Another user had a problem similar to the one you posted and, after hours and hours of troubleshooting, we found out he caused the problem, by manually copying the two GSX packages in the Community folder, when you are only supposed to have Symbolic links to the real installation in the Addon Manager\MSFS folder. So, in addition of wasting twice the space, the manually copied packages were the only thing the simulator had, but that was not only an outdated copy (the Updater only works on the “real” copy in the Addon Manager\MSFS folder ), but he caused a mix-up of different versions, because he had the updated code of the program in the Addon Manager folder, running over an outdated copy of the packages, because he manually copied it.
In brief, trust the installer, and don’t move or copy anything manually trying to fix things.