ERROR,default vehicles_airport.bgl not found!
It means the installer couldn't find the vehicles_airport.bgl file in your FSX\Scenery\Global\Scenery folder, in order to patch it to exclude the default Pushback truck.
Check if you have a vehicles_airport.gsx file in the same folder, that's a backup of the original file made by GSX installation. If you have it, simply rename it vehicles_airport.bgl and reinstall the Addon Manager.
If you don't, then either you manually removed it (you shouldn't have) OR you never had that file when GSX was installed, in that case it should be restored from the FSX installation DVD
It's not really a big issue not having it, you will miss some default vehicles, but this won't interfere with GSX or our sceneries.