Why doesnt my gas gauge work after installing new fuel pump?

Damaged float I expect.

year/make/model?? Does the gas gauge read empty or hard full? If it reads hard full, suspect a ground problem at tank. If gauge doesn't work at all, make sure there is power to the plug at the tank. It is also possible the float arm may have become disconnected as some of these units are tight to get back in the tank. I have also seen new pump/gauge assemblies with defective sending units resulting in non working fuel gauges.

Putting gas has nothing to do with you fuel gauge problem
Must problems to add fuel and comes out can be cause by air flow in the tank, also it is recommended to fuel with the engine turn off so the pressure from fuel pump going back to the tank will not affect the fueling , about you fuel gauge not working properly can be cause by a electrical problem such wiring or defective gauge or can be fuel sender problem with most the time it is install on fuel pump assy.

Has the filter ever been changed? Need to know year and model of the truck. Fuel filters can be near the tank or under the hood. Filters are usually separate from the fuel pump.

You change the sending unit

if it worked for a short period of time after doing the pump/sending unit the check the connections going to the pump/sending unit they would have been moved around doing the pump

Just had a new fuel pump installed in 2002 Chevy Malibu. Worked great for about a week and then the fuel gauge started reading "empty" when the car has a full tank of gas. What happened?

fuel sending unit located on the fuel pump in gas tank

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If your fuel pump is an in tank one then you have probably got the float stuck when you put it in the tank.
Let me know if this is the prob.

Usually if the gas gage does not work yes it is true it may be the sensor if it is electronicly controlled. the sncor will more than likley be in the tank. here is how to remove your gas tank.

1. Before servicing the vehicle, refer to the precautions section.
   
2. Relieve fuel system pressure.
   
3. Drain the fuel tank.
   
4. Remove or disconnect the following:
   
   • Negative battery cable
     
   • Fuel filler and vent hoses
     
   • Fuel tank skid plate
     
   • Fuel tank wiring connectors
     
   • Fuel supply and return lines
     
   • Fuel tank
     
   • Fuel pump assembly
     Fuel pump assembly mounting
     

To install:

1. Install or connect the following:
   
   • Fuel pump assembly
     
   • Fuel tank. Tighten the bolts to 27 ft. lbs. (36 Nm).
     
   • Fuel supply and return lines
     
   • Fuel tank wiring connectors
     
   • Fuel tank skid plate
     
   • Fuel filler and vent hoses
     
   • Negative battery cable
     
2. Start the engine and check for leaks.