I believe
@bkrantz is referring to the method of putting the gauge face down on a towel or soft surface and using a small flat blade screwdriver to work your way around the back of the bezel, twisting the blade to uncrimp it.
Go slow until you get the feel - try a couple of different blades. Eventually, you'll be able to work the bezel off, remove the glass and give it a good clean.
I use compressed air and wear rubber gloves when handling the glass.
Pop the cleaned glass back in the bezel and work it back onto the face of the gauge. Place it face down on a cushioned surface and work your way around, putting the crimp back on. I like a fairly wide flat blade for this but you'll see what works.