In the sense of a standard of measurement, these words are interchangeable (a narrow-gauge or -gage railroad), although gauge is the preferred spelling. Only gage can refer to a variety of plum and to a challenge or pledge: "This is a can of gage (really greengage) plums." "The swordsman threw his glove on the ground as a gage." "This ticket represents a gage at the pawnbroker's."
Dictionary of problem words and expressions. Harry Shaw. 1975.