According to The Columbia Guide to Standard American English,
" ...Gage is a variant of gauge, but apparently in dwindling use..."
For some reason I always thought 'Gage' was an American spelling of the 'English' Gauge.
I have generally seen 'gage/gauge' used to mean an attribute measuring device (GO/NO-GO gage etc.). A variable characteristic is measured with an 'Instrument' (such as a Vernier, micrometer ...).