Replaced gas tank on my (88 camaro) fuel gauge won't read anymore I'm not sure where to start

Well, I'm pretty sure the float, which is connected to that whole fuel-pump part, is very sensitive. if you knocked it around installing it, it could have been damaged. Maybe not likely but possible afaik. The only other thing I can think of is the gauge itself, as the gauges in these 20+ year old cars simply aren't that great.

Did it work just fine before?

Yea it did work just fine but the gas tank had so much rust I replaced it, along with a new engine, and I got the transmission rebuilt. So I'm not sure if any wires weren't hooked up again.. Is there a relay or something we might have forgotten?

Ooh, that's a lot of possible areas that could be causing it. If you can get your hands on a haynes or chilton manual, do so- I can't recall right now what kind of electrical connection goes on between the gauge and the actual pump, but that manual has diagrams that will aid you.