“So how is a Sikh going to put on his gas mask?”
Better question to start with: are surgeons ever issued pro masks? The few times Sikhs have surfaced in the military in my memory, they’re always medical officers who have much needed skills in specializations such as specialized surgery. They can make more money elsewhere and they are often among the best in their profession. Why would a sensible person care about a beard and a hat if it means better care for our servicemembers?
Being in the military isn’t about the individual, or his religious beliefs or his preference for facial hair. In order to optimize unit cohesion the individual has to be subordinated as a whole, with the same haircuts, uniforms and privileges. All this will do is further separate people into individual cliques and cause resentment and charges of either favoritism or prejudice.