When talking about United Kingdom or United States people often pronounce the letters of the abbreviation like "you kay" (UK).
However, I have never noticed anyone spelling out 'GB'. Is it somehow unspellable?
Great Britain is a subsection of the United Kingdom, so it is not as frequently mentioned as the UK is, since the UK is the accepted term. Although some speakers may decide to say "Gee Bee," it is not in common usage.
To be most widely understood, it is best to simply say "Great Britain." I suspect that it has not entered common speech precisely because it is not the official term for the entire country of the United Kingdom.
It is pronounced "Gee Bee" for each letter and is not spelled out, just as "USA" is usually not spelled out.
Most people will be familiar with the old car decals used to designate country of origin
which has now moved on to the license plate
it is also the designation of the Olympic team from the UK
How to pronounce This is team GB