It's worth noting that "Great Britain" is very specifically the name of a geographical land mass -- an island off the coast of Europe that currently includes England, Wales, and Scotland.
In political terms it doesn't include NI ... or the Isle of Man, the Isle of Wight, and the Channel Islands.
(Also, in event of Scottish independence, using "GB" for the nationality would cause confusion because post-independence there'd be two separate nations on the GB land mass. See also [minor] confusion over "Ireland".)