Check your voltages. Disconnect the center wire at the sending unit on the tank. You should have 12V there. Very, very briefly touch that wire to ground, preferably through a 20 ohm resistor. The needle on the gauge should jump up if the gauge is good. If it checks ok, check to be sure the sending unit is grounded. If its good, the sending unit probably needs to be replaced. Thats usually the problem. Its pretty easy to do if you can get to it.
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va