The question that was in the back of my mind here was how did U.S. english usage come to use the TWO spellings, "gage" and "gauge". "Gage" is used in my Brown & Sharpe, Starrett, Pratt & Whitney and Mitutoyo (U.S.A.) catalogues. These catalogues date from the early 1930's to the present. A quick look at my various U.S.A. machine shop texts shows they use "gage" also. My various books from Great Britain, mostly MAP publications, use "gauge". Thank you for bringing this to my attention, Mark. I think it's fair to say that in this country, outside the world of the machine shop, "gauge" is most commonly used.
My 1960 edition of _Webster's New Collegiate Dictionary_ gives both spellings with "gauge" first. And it uses the spelling "gauge" in its illustration of several shop gages/gauges including the feeler, screw pitch, depth, and "go-not-go".
David