Great Britain vs peer of the realm

Great Britain and peer of the realm both are nouns.

Word	Noun	Adjective	Verb	Adverb
Great Britain	Yes	No	No	No
peer of the realm	Yes	No	No	No

Great Britain (noun)	peer of the realm (noun)
an island comprising England and Scotland and Wales	a peer who is entitled to sit in the House of Lords
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