GPO vs federal agency

GPO and federal agency both are nouns.

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

As nouns, federal agency is a hypernym of GPO; that is, federal 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
federal agency: an administrative unit of government

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

GPO (noun)	federal 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