Might be as simple as a little corrosion in the connector. Try finding something to scrape it or if not, just push/ pull it on /off the sensor a few times. To ground try an alligator clip wire inserted into the terminal. Also wiggle the terminal it may be broken inside the molding
Assuming the fuel gauge works, other problems could be in the bulkhead connector, the cluster connector, and the nuts on the studs of the gauge.