If you had a file with a .gsx extension, this is the exact copy of the .bgl file which was found on your system when you first installed GSX.
Whether that file is exactly the original file as supplied by Microsoft, GSX can't obviously know, it's possible the file already had a problem before installing GSX, nor GSX can't know if you renamed the .bgl already patched by GSX to .gsx (which shouldn't be done). In general, if the file has an original creation date of 2006, it's very likely is the original FSX file.
In any case, check your Airport Ground Traffic density slider, maybe it's too low for the pushback truck to appear.
You can search for the original file in the FSX installation DVD, it should be inside one of the .CAB file, so you'll need a compression utility that is able to open MS .CAB files, like Winrar, etc.