mustard gas vs poison gas

mustard gas and poison gas both are nouns.

Word	Noun	Adjective	Verb	Adverb
mustard gas	Yes	No	No	No
poison gas	Yes	No	No	No

As nouns, poison gas is a hypernym of mustard gas; that is, poison gas is a word with a broader meaning than mustard gas:

mustard gas: a toxic war gas with sulfide based compounds that raises blisters and attacks the eyes and lungs; there is no known antidote
poison gas: a gas that is poisonous to breath or contact; used in chemical warfare

Other hypernyms of mustard gas include vesicant, vesicatory.

mustard gas (noun)	poison gas (noun)
a toxic war gas with sulfide based compounds that raises blisters and attacks the eyes and lungs; there is no known antidote	a gas that is poisonous to breath or contact; used in chemical warfare