Gas in a diesel isn't a huge deal on older mechanical injection.
6.5 liter is most likely the size of the engine. For compatibility the military uses the same engines in everything. they used to use the 6.2 Chevy diesel. It was a pig and no one ever liked it, but it was alright. A few years back it was upgraded to the Chevy 6.5 liter diesel. This type of engine has a lift pump, basic Stanadyne/Roosamaster rotary injection pump and mechanical injectors that are not really that fancy. I've rebuilt them, they are simple. If you'd ever worked on the ford 6.9/7.3 it's pretty much the same fuel system (except the pump spins backwards on a Chevy... go figure)
The biggest problem lies in the fact that the pump is relatively fragile regarding fuels on account of it being lubricated and cooled by the fuel. Typically before any real internal engine damage occurs the pump will overheat and seize. This will kill the engine. Typically at this point if you were to drain the fuel and refuel with diesel, it'd start up and run fine, tho you've certainly worn the pump down some (at best they last 100K miles before they start to wear out) On a whole these pumps are not very strong. We used to advance the timing to keep them running, and (dare I be so rednecky?) used to dump cold water on the injection pump to get a truck to start when it was hot.... It was enough to loosen the internals for "just one more run"
With certain pumps like inline pumps (cummins 6bt and the old mercedes diesels used them) mixing gasoline with diesel was common. In the Mercedes owners manual for the old 240 and 300d, it was suggested to mix up to 25% gasoline in extremely cold climates to keep the fuel from gelling. If you ever take these pumps apart you can see that they are much more robust.
