My gas Gage does not work

The most common cause of a gas gauge problem like this is that the fuel level sending unit inside the gas tank is defective and needs to be replaced which is part of the fuel pump "module". To replace the fuel level sensor you must remove the gas tank from the vehicle or lift the bed off the truck. Before you attempt this you will need a good hydraulic floor jack and safety jack stands to support the car while it is up in the air. You will also need a special tool to release the fuel lines from the gas tank outlet nipples.
Here are more complete instructions from a self help site I use.

Instructions

1. • 1Remove the bed of the truck to gain access to the Sonoma's fuel pump module assembly. The fuel module pump flush mounts to the top of the truck's fuel tank. With the bed removed, the pump is available for service or replacement.
     
   • 2Unplug the tail light and license plate wiring harness. With the wiring harness connectors disconnected, remove the plastic harness supports along the bottom edge of the truck bed to prevent pulling them out when the truck box is taken off.
     
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     • 3
       

   • Unscrew the hose clamp securing the fuel tank fill-up line to the fill spout on the truck bed. A standard flathead screwdriver will fit the clamp and provide a means to gently pry the fill line loose from the spout.
     
   • 4Unbolt the truck box by removing the carriage-bolt nuts from underneath the bed. The nuts are manageable with a 3/4-inch or 7/8-inch combination wrench. With all carriage bolts out of the way, get some help from a second person, and lift the bed from the truck.
     
   • 5Disconnect the fuel evaporation lines connected to the fuel sending unit on top of the fuel tank. With the lines removed, unplug the electrical connector, and move it out of the way.
     
   • 6Remove the snap-ring securing the fuel sending module into the gas tank. Use a standard pair of snap-ring pliers to take the ring out of its groove. Keep a second hand over the top of the snap ring to prevent it from flying out and getting lost.
     
   • 7Pull the fuel sending unit assembly up and out of the tank, and set it out of the way. Remove the old o-ring from the hole left by the sending unit.
     
   • 8Lightly coat the new o-ring with petroleum jelly, and insert it into the o-ring groove around the fuel sending unit port on the tank.
     
   • 9Set the new unit into the tank, using care not to pinch or reposition the o-ring. Push the assembly flush with the top of the tank until the snap-ring groove is visible.
     
   • 10Install the snap ring to secure the new fuel sending unit into the tank. Ensure the unit is oriented in the same manner as the old unit before removal.
     
   • 11Connect the electrical connector to the sending unit, followed by the fuel evaporation lines. The lines simply press onto the hose barbs protruding from the top of the sending unit.
     
   • 12Set the truck box onto the truck frame. Install the carriage bolts and wiring harnesses for the tail and license plate lights.
     
   • 13Set the fuel fill-up line into place on the fuel spout, and secure it into place with the fuel line clamp.
     

Could be the gauge on the dash or the sending unit in the tank.