mask vs gas helmet

mask and gas helmet both are nouns.

Word	Noun	Adjective	Verb	Adverb
mask	Yes	No	Yes	No
gas helmet	Yes	No	No	No

As nouns, gas helmet is a hyponym of mask; that is, gas helmet is a word with a more specific, narrower meaning than mask:

mask: a protective covering worn over the face
gas helmet: a protective mask with a filter; protects the face and lungs against poisonous gases

Other hyponyms of mask include face mask, gasmask, respirator, welder's mask.

mask (noun)	gas helmet (noun)
activity that tries to conceal something	a protective mask with a filter; protects the face and lungs against poisonous gases
a covering to disguise or conceal the face
a protective covering worn over the face
a party of guests wearing costumes and masks

mask (verb)	gas helmet (verb)
put a mask on or cover with a mask
shield from light
cover with a sauce
hide under a false appearance
make unrecognizable