It's "gauge", whether it's a strain or a fuel gauge. Or a narrow gauge railroad, or a 12-gauge shotgun.
In my personal opinion it should be "gage" in all these cases because it is definitely not spelled the way it is pronounced. But who cares what I think?
"gauge" is the correct spelling in most cases, but "gage" does get used in technical situations, as Dictionary.com indicates.
Here's Dictionary.com's version. The spelling "gage" is an occasional variant, but mainly for technical use (see gage). Gage is largely an archaic spelling, and in one archaic use, a gage is "something, as a glove, thrown down by a medieval knight in token of challenge to combat."