Don't forget the military ATCOs are unable to join a union and it is certainly the case that GATCO offers some great benefits and professional backing that most mil ATCOs (and Fighter Controllers) are glad to have in their back pocket. I think that GATCO must therefore be the only ATCOs organisation that truly represents ATC UK-wide.
Also, I have done some work alongside GATCO in the past by way of promoting flight safety and education amongst the GA community. They were thoroughly supportive of the aims and lent their weight to the events.
I've been a member of other professional bodies in both Science and Management; GATCO is run on a voluntary basis by ATCOs for ATCOs and gives me much more for my money.
SID
