You must first fix your P3D registry key to point to the correct path P3D is installed into, and then GSX will start working again, you might not need to reinstall it but if you do it, the installer will also work correctly, like any other 3rd party addon installer which is now likely to be confused by the wrong registry, until you fix it.
If you open REGEDIT and go to that key, it would be very easy to see that you must edit the AppPath key, which is a string containing the name of the folder P3D is installed into.