If the instrument VR is working you should see voltage at the gauge. With a stock VR it may be pulsing 12 volts, or with a solid state regulator, about 5 volts constant. Either way, with the sending unit wire grounded it should read Full or above. If it does not read at least FULL the problem could be the gauge or the instrument VR output. Easiest thing would be to swap out either item with a known good one. If the Voltage regulator is good, you may be able to re-calibrate the Fuel gauge by slightly moving/rotating the small teeth behind the small holes on the back of the gauge?
If the gauge responds when the sending unit wire is shorted to ground, but not when connected to the sending unit, make sure the sending unit is grounded. The factory used a small strap that cliped to the fuel line comming out of the tank jumpering to the cars chassis mounted fuel line.