[71GMC_3/4T]

I'd start with trying to track down where it got broke in the wiring harness. Disconnect the wire. Measure the resistance between the disconnected wire and ground with the key off. It should be nearly open. If it's shorted, there's your problem. Also, see what the guage does with the sender wire disconnected. I can't tell you what it should do without going out to my newer Chevy and seeing what it does - sorry.

Guages usually don't go bad. If it goes back down to somewhere reasonable with the key off, the wire is probably shorted to ground, most likely in the engine compartment, but hey, who knows, maybe some radio tech hooked into that wire for a ground by accident?