Indeed, Great Britain is the island containing England, Wales, and Scotland, and the UK is Great Britain plus Northern Ireland. But GB is the ISO 3166 code for

Indeed, Great Britain is the island containing England, Wales, and Scotland, and the UK is Great Britain plus Northern Ireland. But GB is the ISO 3166 code for the UK, including Northern Ireland. The reason is that the ISO committee wanted to avoid generic words like "united" and "kingdom" so they went for the more "unique" GB, in spite of being somewhat politically sensitive, confusing, and inconsistent with some other standards and every-day usage.