gulag vs prisoner of war camp

gulag and prisoner of war camp both are nouns.

Word	Noun	Adjective	Verb	Adverb
gulag	Yes	No	No	No
prisoner of war camp	Yes	No	No	No

As nouns, prisoner of war camp is a hypernym of gulag; that is, prisoner of war camp is a word with a broader meaning than gulag:

Other hypernyms of gulag include internment camp, POW camp, prison camp.

gulag (noun)	prisoner of war camp (noun)
a Russian prison camp for political prisoners	a camp for prisoners of war