Gas Gauge

If the other 2 gauges work thats a good sign.
First look for the ground strap at the sender . It snaps on the metal fuel lines conducting ground over the rubber hose there. It may be missing.
Next attach the sender wire directly to ground and see if the gauge needle moves. It should peg full high as it can go. Dont leave this hooked up and hot for longer than 2 minutes or it may fry the gauge.
Let us know what you find .