94 Bonneville gas gage screwed up?

I just bought a 94 Bonneville off of my mother so i know the car pretty well. Recently it started having gas mileage problems, so i took it to the shop for a
tune up and a new fuel filter. I checked the fuel tank for leaks and there are none. When I start the car the gas gage goes up and down then goes way down, even after i put gas in it. is it the gas gage if so what do i do?
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