gas/fuel guage not working

They most likely got the fuel tubes wrapped around the fuel pump's floater. Theres an L Shaped bracket on the fuel pumps that floats up and down with the fuel level, this sends a signal to the fuel gauge and then it shows ya the level. Occasionally when installing pumps people accidently get the floater hung up in the fuel hoses. Sounds like thats what happened to you.

On the doing it yourself part? Sure, you could do it. You'll need the car jacked up and on stands, and a jack underneath the gas tank. 3 bolts hold the tank up with the metal straps. Only one strap comes down the other is hinged. Support the weight of the tank with the jack (easier when it's not full, and slowly lower it. Theres a round cover in the top center of the tank with a bunch of small bolts on it. Unbolt those and pull the cover off. Be careful not to loose any of the bolts or drop them in the tank. Also, the fumes will be overpowering. Make sure you have plenty of fresh air and dont put your face directly in there if your huffing and puffing. Pull the pumps out, taking care not to bend the floater and then reseat it. Should be fine after that. However, since you paid to have a job done... I'd take it to them to correct thier mistake. Otherwise they can say you messed it up further by doing it yourself if something still isn't right and will charge ya for the parts to correct it.

definately the float