Results of search, Courtesy Mc Loud
http://www.thegsresources.com/garage...ide_gauges.htm
I did this a few years ago, before the cut-off suggestion was made, but it may be a better way.
My way was rather brutal, as it used the second worst implement in the tool box: a pair of vice grips. ( a Hammer might be worse)
I wrapped duct tape all around one jaw of the pointed vice grips. This was intended to avoid scratches to the case.
The vice grips were then locked in a bench vise.
With the covered jaw against the outside, I gripped the ring lightly with the front tooth of the vise grips and bent it SLIGHTLY backwards.
After that, holding a slight pressure, the gauge was turned against the vice grips a full revolution, then bent a tiny bit more, and turning repeated It came clear on the third turn.
Closing it was easy....Pressing the shank of a screwdriver against the turned edge and going all around it, with just a bit of pressure, then repeating this until it was in place. You can see the bend marks in the metal, if you look for them, but the seal and glass went back into place and never leaked.
This worked, but if you wish to do it yourself, you should note that patience is required, and it must be done slowly, because using too much pressure at any one time will break the glass.
Again, the cut-off suggestion seems to be a good idea, and probably has less risk.