Coolant temp gauge problem

I have grounded the wire to the block and the alternator case since they're close by. I attached a long wire to the connector in the engine bay and ran that into the cab. I can ground that to the dash frame and watch the gauge go up to hot.

I don't have any sealant on the sending unit.

The gauge doesn't have a ground. It only has two posts on it, one is 5V supply the other is the wire that goes to the sending unit. I thought the sending unit supplies ground to make the gauge move.