I stand to be corrected but I think this breaks TUC rules so the GMB could be in big trouble, not to mention that at least one of the people they have recruited as a "rep" seems to have absolutely no idea of the principles of trade unionism. I expect this will implode and some people will be very embarrassed. I will watch with interest.
"All affiliates of the TUC accept as a binding commitment to their continued affiliation to the TUC that they will not knowingly and actively seek to take into membership existing or “recent” members of another union by making recruitment approaches, either directly or indirectly, without the agreement of that organisation."
TUC rule book