I don't think the GMB have been campaigning for Unite members to join.
This has been set up for non union members and thoes that feel unable to join Unite/BASSA.
Individual cabin crew have been suggesting people join the GMB but the GMB website clearly is directed towards people not already in Unite/BASSA.
I don't think it would be very democratic for Unite to say that people can't be in another union. Quite frankly many who are joining the GMB would be quite happy, being members of Unite, they just don't want to be in BASSA.
So there lies the problem!