Great Britain vs Britisher

Great Britain and Britisher both are nouns.

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

Great Britain (noun)	Britisher (noun)
an island comprising England and Scotland and Wales	a native or inhabitant of Great Britain
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