If you take the wire loose at the sending unit does the gauge go to full? Or if you take the sending unit wire loose from the back of the gauge does the gauge go to full?

If it goes to full when you disconnect the wire at the sender then the sender is grounded. If it goes to or past full when you disconnect the sender wire at the gauge then the sender wire may be grounded some where along the way to the tank.

Could it be the power wire to the gauge is on the wrong terminal?