The backup is made BEFORE touching the original file. The file is not renamed, is copied to .GSX and the other copy named .BGL is patched to disable the default Pushback truck, and the installer clearly alerts it did this, and says it made a backup too.
And, if no vehicles_airport.bgl is found when installing, but a vehicles_airport.gsx IS found, the .bgl is first restored from the .gsx and then patched.
When you uninstall GSX, the backup is automatically copied back to its original file, so you'll end up in exactly the same situation as if GSX wasn't installed.
Note, this is only using the *current* version of the Full 1.4.1 installer, or the most current version of the Stand-Alone Addon Manager.
Previous versions of the installer only worked with an original vehicles_airport.bgl, not one that was already modified by another product (there are 3rd party vehicle replacement sets), but that only meant the patching did't happen, so you would end up with double Pushback trucks. The current installer can patch any vehicles_airport.bgl file, even a non-original one.