Great Britain vs gun room

Great Britain and gun room both are nouns.

Word	Noun	Adjective	Verb	Adverb
Great Britain	Yes	No	No	No
gun room	Yes	No	No	No

Great Britain (noun)	gun room (noun)
an island comprising England and Scotland and Wales	military quarters of midshipmen and junior officers on a British warship
a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland; `Great Britain' is often used loosely to refer to the United Kingdom