i put 20 dollars in gas yesterday and it gave me just over half a tank. i drove 60 miles and its still at half a tank. i dont believe this at all. how do i determine whether the gas gage is bad or not?

You could do a complete top off, reset your tripometer and watch your mileage. Good chance your sending unit is going bad.

if it is the sending unit how do you replace it?

You would have to drop the fuel tank and pull the whole unit out. If you did do this, it would be in your favor to replace the sending unit and fuel pump at once.

Considering the age of your car, your fuel tank straps and mounting hardware are pretty rotted, replace those too.

Here'* how to get the tank off https://www.gmforum.com/mechanical-134/replacing-fuel-pump-92-99-a-282006/

I have a bad gas gauge too--It will sit either at full or empty. I just use the tripometer.