"ascertain by exact measurements," mid-15c., from Anglo-French gauge (mid-14c.), from Old North French gauger (Old French jauger), from gauge "gauging rod," perhaps from Frankish *galgo "rod, pole for measuring" or another Germanic source (cf. Old Norse gelgja "pole, perch," Old High German galgo; see gallows). Related: Gauged; gauging. The figurative use is from 1580s.
"fixed standard of measure," early 15c. (surname Gageman is early 14c.), from Old North French gauge "gauging rod" (see gauge (v.)). Meaning "instrument for measuring" is from 1680s.
A shotgun: a shotgun is called ''the gauge,'' explained Officer Phil Lee/ This man took a gauge (Armond pantomimes holding a gun, then bends over to dodge from it) and two people end up dead
[1970s+ Underworld & police; fr the use of gauge to designate the caliber of a shotgun]