the sender IS the ground, the gauge gets + only .... if you have 12V showing at the [disconnected] sender wire to ground with the key off, you've wired something wrong - gauge power comes from the ignition switch so there should be no power there until you turn the key on
the way it works is the gauge gets a variable resistance to ground as the float moves, changing the current thru the gauge to move the needle .... look at your wiring diagram and recheck your wiring job .... also check the sender itself with your ohmmeter to see if the resistance varies from about 0-30 ohms as you move the float arm and make sure the tank is grounded so the sender is getting a good ground
Bill
