An electrical gauge uses a "sending unit" to send the info to the gauge. Example: oil pressure gauges come in two atyles, mechanical, and electric. A mechanical oil pressure gauge uses a piece of tubing to plumb between the oil galley in the block, and the back of the gauge, to read the oil pressure. Good point; a very accurate gauge, bad point; hot oil is plumbed into the interior of the car, and if the tubing fails, it makes a huge mess and a hazard to the occupants of the car. An electrical gauge will have a sending unit at the oil galley on the side of the block. It reads the pressure, sends the info via electric wiring, to the gauge. Much safer this way. I hope this explains to you what a sending unit is. As far as the fuel gauge is concerned, it reads off of the float that is inside the gas tank. The float has a sending unit on it, but I do not believe that it can cause a leak. The entire float/pump assembly may be leaking at its mounting point on the tank, but it should only leak when the tank is pretty much full of gas. If you have a leak, even when the gas tank is less than half full, then your float/pump assembly is not the cause. Now it may be fuel lines, that would make more sense.