Well, I count 3 things that it could be:
a break in the circuit from the tank to the cluster
a faulty gauge
Since your testing with the volt meter indicated that the sending unit is working properly (variable resistance between 0 and 90 ohms) we can cross that off the list. If it is the gauge, the entire cluster would have to be replaced. Before you incur the cost of a replacement cluster, I'd think that you would want to be sure that the same resistance readings he is getting at the tank are seen at the cluster connector. This will verify that the circuit is OK.
If the correct resistance is seen at the cluster connector, replace the cluster.