2002 Pontiac Grand Am 2002 Grand Am Gas Gauge off

Interior problem
2002 Pontiac Grand Am 6 cyl Automatic 110000 miles

I bought this car on Thursday, when I purchased it the gas gauge read 1/4 tank so I stopped to fill up. I put $20 in and pulled away and the gauge only went up a hair. I was confused, but thought maybe it just had a huge tank. I stopped Friday to put more gas in, got to $5 and the pump clicked indicating it was full. Left the station and the gauge was at 1/2 tank. Drove 3 hours south of my home and mileage was at 230 so I stopped to get gas again because the gauge was at 1/4 tank. $16 and tank was full. Back at 1/2 tank on the gauge. Drove 2 hrs north to go back home and all the sudden the gas gauge just dropped to E and the gas light came on. I freaked thinking there might be a sensor that causes the car to cut off after so many miles with the light on so I stopped at got gas $5 and it popped.

I am so frustrated because I have no clue how much gas I have, I was just gonna go by miles and fill up every 200 miles, but after that scare with the gas light coming on I don't know what to do! I "thought" I got a good deal on the car but when I got home with it I noticed many many things were wrong. All small things, but there were bulbs and fuses burnt out on the drivers side which coincidentally I think had been in an accident because the fender and door have been replaced and the bumper repainted. I had to replace almost all the bulbs and fuses for the drivers side. Could this be a fuse for the gas gauge? All the other gauges work. HELP!