Pretty sure the gauge is grounded as part of the cluster... and the cluster has a couple of grounds iirc but they all lead to a "star ground"... a bronze disk on the driver's side near his knees under the dash with about 15 brown wires leading to it.
I'm also pretty sure the fuel gauge and temp gauge share the same ground... since they share the same voltage stabalizer... so it's perhaps more likely the issue is a circuit board problem, a problem with the wiring between the sensor and the cluster, or with the gauge itself.