peer of the realm vs Great Britain

peer of the realm and Great Britain both are nouns.

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

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