A few months ago I performed a live update and to my surprise, pushback tugs appeared at all the gates at my airports alongside the baggage trucks and conveyers. I'm talking about the vehicles that are generated at the gates and service the AI aircraft, not the GSX service vehicles.
Since you are not referring to the GSX vehicles, this cannot be caused by a GSX Live Update. The default Pushback truck are disabled ONLY by the GSX installer, and for good reason, so you won't see a DOUBLE Pushback truck, which might also conflict with the GSX one.
What the GSX installer does, to remove the default Pushback trucks, is to patch the default Scenery\Global\Scenery\vehicles_airports.bgl file to disable the trucks.
This is done ONLY by the GSX Full installer, the Live Update never touches that file, since it assume it has already been patched by the GSX installer.
I attributed the change to the GSX update because I had not made any other changes to my FSX Acceleration setup other than the live update.
No, the "change" never had anything to do with the GSX update. What really happened, is that you have restored the default vehicles_airports.bgl file. You or something else, like another program. But there's really no other explanation: if the default trucks reappeared, it means the default vehicles_airports.bgl which GSX patched, has been restored.
Uninstalling GSX, will also restore the original file, of course.
However, when I started FSX, the first thing I noticed was that all the pushback tugs were now gone! The baggage trucks and conveyers are there but sadly the tugs are gone again.
This is entirely normal, and it's what is supposed to happen. If you don't mind having double pushback trucks at your gates, you can restore the original file, since GSX made a backup, named vehicles_airports.gsx.
If you copy it over the vehicles_airports.bgl, it will revert to default.
