A gas mask will not seal over a beard. It will let the poison in. Also OBAs, Oxygen Breathing Apparatus, (air for damage-control firefighters) won't work with a beard.
Back in the '80s the Navy decided beards were out. I was a Marine in a Marine Detachment at that time. You never heard such bitching and moaning in your life!
Some Sailors retired, some took discharge.
The Navy is still in place and still operating.
I'm sorry that this Sikh can't join up, but I'd be a damn sight sorrier if he died from poison gas because he wouldn't shave.
I was a Coastie when the no-beards rule came down.
Some of us sent our beards in envelopes to the Commandant. But we shaved 'em.
Similarly, there was a case at a civilian oil refinery, wherein a Sikh lost his job for refusing to shave his beard, thereby making a safety-mandated gas mask policy useless for him.