My gas guage will not work on my 1991 buick riviera what do i need to fix it?

About 98% of fuel gauge issues are the fuel sender in the gas tank. This is part of the fuel pump assembly, and is replaced with the fuel pump. There is another place to check though. The wiring that goes to the fuel sender can get unhooked (possible, but not often happens). The connector is above the gas tank; either way you would have to take the gas tank down. By attaching a rheostat or variable resistor, you can test the wiring and gauge system to see if it works. If it does, then you know that you need to replace the pump to get it working again. If not, then you have wiring issues between the pump and the dash. Hope this is what you're looking for.