How can you disable the speed governor on a used GMC hertz truck?

Depends on what it is. If it's a van, like the G3500, you'll have to get the ECM reflashed at a dealership with new parameters put into it. If it's something like a Top Kick, it'll typically have either a Cat or Cummins engine... you find a truck service center which has the corresponding software, they'll plug in to the engine ECM through the SAE J1939 port, and disable it that way... don't remember the process for Cat engines that well, but it's easy with Cummins engines and the Insite software... you literally go to "Features and Parameters" and select "disable" from the drop-down menu next to where it says "Road Speed Governor".