gauge, gage

Gauge or gauge means to measure or to estimate the measure of--
He spent several minutes attempting to gauge the strength of his opponent.
Gauge may mean a particular standard of measurement--
The narrow gauge train could not run on the wide gauge track.
Gauge may also mean an instrument used to measure something--
According to the rain gauge, we have had more than an inch of rain.

Gage or gage ( rhymes with gauge, sometimes used to mean gauge ) means a glove thrown down in token of a challenge, as to a duel--
When the knight threw down his gauntlet as a gage, he meant to challenge his foe to combat.
Gage also means a token of a pledge or pawn--
He left his watch as a gage that he would return to pay the loan.

Now that you know that, you can say--
"Leaving his guitar as a gage in the pawn shop was a gauge of the musician's desperation."

Note--gage may also mean a type of plum ( fruit ), more commonly called a "greengage"--
How can you tell when a greengage is ripe?
Note # 2--"Gage" ( always capitalized ) may be a proper name--
General Gage left America soon after the Revolution began.