...without specifying your DMM techniques, find out why there is no indication on the gauge when you ground the body of the sender. Verify that your ignition is 'on.' Wire brush the sender threads to remove any coatings that may interfere with your ground. Remember, a gasketed oil filter billet may be intermittent to ground. It depends on bolt head contact with the body and thread contact with the engine. Make sure bolts are secure into block...despite that the other gauges are functioning.
If the sender gives a 100 ohm value when out of the mount (I'd recheck this.) the gauge should indicate. Otherwise, there is an intermittent preventing it from registering. If you're still stumped, wrap or clip a wire from the body of the sender and connect to a known ground. If it works properly, then the mount is suspect.