Great Britain vs Briton

Great Britain and Briton both are nouns.

Word	Noun	Adjective	Verb	Adverb
Great Britain	Yes	No	No	No
Briton	Yes	Yes	No	No

Great Britain (noun)	Briton (noun)
an island comprising England and Scotland and Wales	an inhabitant of southern Britain prior to the Anglo-Saxon invasions
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	a native or inhabitant of Great Britain

Great Britain (adjective)	Briton (adjective)
	characteristic of or associated with the Britons