Check and see if your year model is under GM's instrument cluster recall. My 2004 bad cluster made the gas gage read incorrect also. GM claimed it was the fuel pump sender, but I made them put in a cluster. I was right, fixed the problem.
If it is the pump sender, the pump+sender only cost about $200 aftermarket.
Lift the truck bed and they are easy to replace. Most GM pumps stop pumping about the age of yours. Took a couple of hours on my suburban since I had to work from under it.
