Gas gauge not working

Either the gauge or something in the cluster, including the IVR, or a poor wiring / connector problem. You just need to dig in. One thing you can do is identify the sender wire in the left kick panel rear harness connector and jumper that to ground.

Are you sure the temp gauge works OK and is fairly accurate?

Shorting a sender wire to ground should result in an IMMEDIATE needle over to the far far right. You don't want to leave the key on any longer than necessary.

You have a factory shop manual?