I agree with all of you; at least in part. Here's my tuppence worth.
First, I agree with HannibalII's first response that, in the particular circumstances of the question, "Great Britain" should have been used. I say that because all the individual countries were named in full, so the collective term for them should also have been named in full.
In other circumstances, I agree with Odessa Dawn and Tdol that it's perfectly fine to use the abbreviation "GB" - so long as the context is appropriate, and only these 3 countries are included. Unlike HannibalII, I do see "GB" used, but all too often incorrectly when the context includes Northern Ireland. In these circumstances, the term "UK" should be used. Because the "United kingdom of Great Britain and Northern Ireland" is such a mouthful, it is almost never quoted in full, and "UK" is used almost exclusively.