GAO vs government agency

GAO and government agency both are nouns.

Word	Noun	Adjective	Verb	Adverb
GAO	Yes	No	No	No
government agency	Yes	No	No	No

As nouns, government agency is a hypernym of GAO; that is, government agency is a word with a broader meaning than GAO:

GAO: an independent nonpartisan federal agency that acts as the investigative arm of Congress making the executive branch accountable to Congress and the government accountable to citizens of the United States
government agency: an administrative unit of government

Other hypernyms of GAO include agency, authority, bureau, federal agency, office.

GAO (noun)	government agency (noun)
an independent nonpartisan federal agency that acts as the investigative arm of Congress making the executive branch accountable to Congress and the government accountable to citizens of the United States	an administrative unit of government