The gauge ground is through the variable resistor in the tank. The only thing that would require the gauge case to be grounded would be for a dash light.
The fuel gauge is nothing more than an ammeter and works the same as an ammeter when wires are reversed, except it can't go below zero when reversed like the ammeter can.
It sounds as though you have not tried to disconnect the wire to the sending unit at the gauge and grounding that terminal yet. It is necessary to do this for diagnosis. The five volts you read at the tank would probably indicate about a third of a tank, and could mean that the tank unit is not working correctly, but check the gauge first.
