Reserve gage for the legal sense meaning, as a noun, a valued object deposited as guarantee against an obligation (hence mortgage), and, as a verb, to offer something as guarantee against an obligation. Use gauge for all other purposes: as a noun, an instrument that measures magnitude, amount, or contents of something, and the thickness, size, or capacity of something; as a verb, to estimate or determine the magnitude, amount, or volume of something, and to measure the dimensions of something with a gauge.
Avoid misusing gage for gauge or misspelling the latter as guage.