gas helmet vs gasmask

gas helmet and gasmask both are nouns.

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

As nouns, gas helmet and gasmask are synonyms defined as:

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

Other synonyms of gas helmet include respirator.

gas helmet (noun)	gasmask (noun)
a protective mask with a filter; protects the face and lungs against poisonous gases	a protective mask with a filter; protects the face and lungs against poisonous gases