1997 Chevy Monte Carlo Gas Gauge?

1997 Chevy Monte Carlo 6 cyl Automatic

Okay. For Christmas my father will be buying me a new gas tank for my 1997 Monte Carlo, I already knew the gas tank had holes towards the top, evidenace the car had sat with a half tank of gas, and got holes at the top. My question is, The gas gauge its not working properly. When I first got the car, it kinda worked, not wonderfully but it did for the most part, with the holes in the tank, so I don't think thats my problem. I am just wondering if I we're to need to get something to replace someone within the gas tank, I would want to put it in when we are putting in the new gas tank, my therefor my question is, what would I replace to get the gas gauge working properly. Would it be the fuel pump, sending unit(however on sending units, I couldn't find anything above 1996) or fuel pump molecule, and would it be expensive? Please help thank you very much.