fuel gage shows empty

GM trucks have had issues with their wiring to the fuel sending unit in the tank. When you put the truck in gear, you actually change the ground path and you may be helping the signal from the fuel tank get to the gauge, as strange as this may sound. So have a good electrical person inspect your wiring harness to your fuel tank, there is most likely a poor connection somewhere. http://repairpal.com/chevrolet-repair-in-galt-california