What "issue" ? The program is asking to activate, because you uninstalled GSX, but haven't uninstalled the Addon Manager so, it sees you STILL have a Serial Number in your registry, but you haven't activated it yet so, of course, is asking for you to reactivate it.
If you were planning to uninstall GSX and not reinstall it again on this PC, you should have selected "Deactivate", which you can do now, even if GSX is not installed.
It would be wrong if GSX deactivated itself if you "just" uninstalled it. If it did, you would have to reactivate it again when reinstalling it, which is of course very annoying, since a new activation is required ONLY if you reinstall Windows or change your hardware.
What would be the point of forcing to reactivate at each Uninstall/Reinstall on the SAME system and OS ?
That's why there's a separate Deactivate function, which you can use if you KNOW you are not going to use GSX again on this system. Which will also reclaim an activation, so it won't consume a new one when reactivating on a new one.
Really, I don't understand why this could be thought as an "issue", when in fact it's a great feature: it allows you to Uninstall/Reinstall an UNLIMITED amount of times (even reinstall the whole simulator), without having to worry about reactivation, as long you don't change hardware or reinstall Windows.