Noticed a problem when was trying to add a new band from GB. There were no GB on the list of countries.
Is that some part of the rules or something? I also did not find anything on the forums about that (i used the search mechanism).

Sorry, i just found that GB here is called UK.

Terminology varies, but Great Britain is generally/officially/correctly not used to refer to the nation but the island (or Wales, Scotland and England together as a political unit). Obviously, United Kingdom is the term we use, as we go by the ISO standard. It's also the more accurate term, generally speaking.

Charming that you'd think we went over ten years without it, though.