nerve agent vs poison gas

nerve agent and poison gas both are nouns.

Word	Noun	Adjective	Verb	Adverb
nerve agent	Yes	No	No	No
poison gas	Yes	No	No	No

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

nerve agent: a toxic gas that is inhaled or absorbed through the skin and has harmful effects on the nervous and respiratory system
poison gas: a gas that is poisonous to breath or contact; used in chemical warfare

Other hypernyms of nerve agent include agent.

nerve agent (noun)	poison gas (noun)
a toxic gas that is inhaled or absorbed through the skin and has harmful effects on the nervous and respiratory system	a gas that is poisonous to breath or contact; used in chemical warfare