You need to check the circuits at the sending unit connector. The sending unit is in the gas tank. The wiring connector is on top. The wiring diagram for connector is below. If you unplug the connector, use a resistor between the two wires shown. Key on, just till the dash lites come on, no need to crank or start. 90 ohm resistor, the gage should read empty. 12 ohm resistor, the gage should read full. You can get resistors from radio shack, just cost a few cents.
You have voltage and ground at the connector? If the gage reads properly with the correct resistor, suspect the sending unit in the gas tank. If the gage doesn't read properly, then could be issue with wiring circuit or instrument cluster.