Check fuse number 2 it is a 7.5 Amp fuse that supplies power to the gas gauge and speedometer with the ignition key on. It also supplies power to the Water temperature and RPM gauge so if they work they fuse is probably OK.
A speed sensor on the transmission puts out an a/c voltage that increases as the vehicle speed increases (the sensor has red/yellow and red wires attached to it). Grounding the green wire at the fuel tank sender unit momentarily with the ignition key on should make the gauge read full if the gas gauge has power and the gauge and circuit is good back to the sender unit in the tank.
Any repair shop will charge diagnostic time to investigate the problem with each prior to repairing parts.