Great Britain vs post-free

Great Britain is a noun but post-free is not a noun.

Word	Noun	Adjective	Verb	Adverb
Great Britain	Yes	No	No	No
post-free	No	Yes	No	Yes

Great Britain (noun)	post-free (noun)
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

Great Britain (adjective)	post-free (adjective)
	postpaid

Great Britain (adverb)	post-free (adverb)
	having the postage paid by the sender