It depends, if its an older model and its an anatomic design (ie) not oil filled, yes you can open it.!
Even if its oil filled it can often be opened, but refilling with oil is an issue - and another thread.!
Best is to remove it from the housing, soften the rubber by imersing it in a little hot water first, this makes it easier to remove and later replace the module.
Next, take a peek at the bottom of the gauge, if it has a black bezel ring, its possible to open.
First, mark the position of the bezel ring in relation to the case with a bit of paint or a permanent marker, next unscrew the bezel ring (its usually not too tight) COUNTING THE TURNS by the marks - its important to return the bezel ring to the same position on fastening down.
Underneath you will find a rubber diaghram and usually a small steel plate, remove them and see if the needle has returned to its zero position - if yes, they were probably stuck or sandy, wash them off and put it all back together.
If not, the mechanism is at fault, all is not lost though, look carefully for rust due to leaks, if its rusted its usually done, keep it as a memento and get a new one.
If theres no rust its just sticking, move the mechanism by the small arm which rests on the steel back plate and see if the needle moves freely, if yes, lubricate the mechanism with a SMALL drop of thin oil - dont use an aerosol, its just overdos everything - test again, usually thats all thats needed and you can fit it all together again.
Now TEST the gauge against another known good one on a dive to check calibration, its unlikely it will go out if the bezel is returned to the exact position, but ALWAYS check first.
Its always fun to fix it yourself, but it always go's without saying NEVER,NEVER work on gear if you are in any sense unsure of yourself - rather ask a professional - although from my experience few will be bothered to fix a gauge.