it seems GSX randomly selects the push-tug. Its nice to have the option.
The selection is not really random. As explained on the GSX manual, Page 16, when the tow truck is discussed, they are selected depending on the airplane weight. They are randomly selected again, in case there are multiple candidates that match the airplane weight limitations for those models.
They are not regionally-encoded by default but, of course, nobody prevents you to use the documentation in the PainKit to learn how to modify the icao codes and specify that a specific model should only appear on certain areas/airports/countries.
In fact, two pushback trucks ARE regionally coded, there's one from Aloha that will only appear in Hawaii, and a blue KLM model that will only appear in the Netherlands so, you might start from there and add your own repaints, just look the Simobjects\Misc\FSDT_Pushback\SIM.CFG file to see how they are organized. Everything is explained the PaintKit in more detail, of course.
