Ok, the code for the light would probably would have been, either P0455, P0446, P0449 or P0440 which refer to evap leak. which means the fuel tank is not holding the desired amount of fuel vapors. common causes are gas cap, vent sol. stuck open,purge valve and maybe a hose that is leaking fuel vapors. now the fuel gauge problem is most likely is a fuel level sensor in the tank, which will sometimes turn on the ses light and leave a code P0463. it may be a good idea to have the system scanned for codes. some of the auto parts will do free. like auto zone and some others. if you could have the system scanned for codes and post me back. I could give you a better idea what to repair. pretty sure you have a fuel gauge sensor in the tank going bad.
now the voltmeter, same goes here. have the electrical system checked. find out what the alt is putting out. amps and voltage.