I'm not second-guessing you at all Rhinoman. I just think of the entire block as ground and if you screw something metal into it, that is ground too - the entire electrical system on the automobile is common ground. I just wouldn't have thought the gauges would have any measurable effect on each other because they have a common ground.
Fortunately, I am not using a 12V gauge either - it's mechanical so it is not supplying a voltage that could interfere anyway.