GB vs Great Britain

GB and Great Britain both are nouns.

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

As nouns, GB and Great Britain are synonyms defined as:

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