Agree. ^ If all the wiring is good and not shorted and the gauge checks good, then the sender unit needs to come out of the tank.
With the key on, take the single brown wire off the sending unit on the tank and touch it to ground. The needle on the gauge should read empty. If you unplug the sender wire from the fuse panel the gauge should go towards the full position.
Another possibility is the sending unit may be stuck or float/cork leaking and not working.
Here's the full info on testing the sending unit.
http://www.6066gmcguy.org/Electrical-02.htm