The guard gauge is a (usually invisible) meter that measures guard crush values accumulated by someone who blocks.
Basically, it starts out full, and loses some of its value every time a player blocks. When it has been whittled down to zero, the player’s character will experience a guard crush. It does, however, regain its value over time, little by little.
Most games do not actually show the guard gauge as an actual gauge, but will still give a visual warning that it is getting low (such has having the character or their lifebar blink).