If the tank is full, that's what I'd expect to see from the gage. If the tank is empty consider this. The gage reads full with zero resistance to ground on the signal wire. When the float rises up with filling the tank the resistor in the sending unit decreases resistance. Zero resistance to ground could be a short in the signal wire from the tank, float stuck at the top of it's travel, or the resistor inside the sending unit has an electrical short. Double check that the wire going from sensor to gage isn't the ground reference wire instead of the signal wire. Good luck.