Great Britain vs in

Great Britain and in both are nouns.

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

Great Britain (noun)	in (noun)
an island comprising England and Scotland and Wales	a unit of length equal to one twelfth of a foot
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

Great Britain (adverb)	in (adverb)
	to or toward the inside of