GB vs Britain

GB and Britain both are nouns.

Word	Noun	Adjective	Verb	Adverb
GB	Yes	No	No	No
Britain	Yes	No	No	No

GB (noun)	Britain (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
a unit of information equal to 1024 mebibytes or 2^30 (1,073,741,824) bytes
a unit of information equal to 1000 megabytes or 10^9 (1,000,000,000) bytes
a highly toxic chemical nerve agent that inhibits the activity of cholinesterase