Fact you have the .gsx backup file doesn't mean the default vehicles .bgl airport has been successfully patched now, it only means it has successfully patched when you it was still default.
Once you modified it, it can't be patched anymore so, you'll see double Pushback trucks, which is not usually a problem (other than a cosmetic one), since GSX pushback truck is not affected by the default one, and it starts in a different position.
There's no way to get rid of the default pushback vehicle other than patching the file, you just can't remove some file. If you remove the airport vehicles bgl, you'll lose ALL default ground vehicles so, if this is ok for you, you can just do that.
Or, you can remove the ground traffic complexity slider, so it will not display ground vehicles anymore.
Just to be more clear: the whole issue is just a visual thing: regardless if you see or not the default pushback vehicle, it will not affect GSX Pushback *functionality* in any way.