Gauge mode is when you turn off the algorithm in the dive computer that does NDL calculations for you, and instead it only displays your current depth, your max depth so far, and the current dive time.
This is used by divers (1) who are using mixes that the dive computer is not disigned for, such as trimix, or (2) who are using deco software other than the algorithm built into the computer, or (3) they have learned to become independent of dive computers, and instead they keep a running NDL program in their head (normally that kind of fly-by-wire is also done in conjunction with nitrox).
With the Suuntos, they keep a running dive profile for you, even in gauge mode. That is what makes them really nice to have, superior to the simple bottom timers. Bottom timers are like a combination of a depth gauge and a diving clock, with no NDL nor deco algorithm.