GPO vs government agency

GPO and government agency both are nouns.

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

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

GPO: an agency of the legislative branch that provides printing and binding services for Congress and the departments and establishments of the federal government
government agency: an administrative unit of government

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

GPO (noun)	government agency (noun)
an agency of the legislative branch that provides printing and binding services for Congress and the departments and establishments of the federal government	an administrative unit of government