We have the same arguments here in Canada between pro-gun groups.
The more the merrier, sez I. More pro-gun groups gives the appearance of a groundswell movement, which politicians fear. Multiple organizations also keep each other on their toes.
As a Canadian gun owner, I do not want a single organization representing my interests. Even though the organizations tell me how good that would be, I know human nature. Power corrupts. Gun control is as much about statism as it is about guns. Why would I want to create an oligarchy to protest the notion of an oligarchy?
I believe the GOA was founded by a U.S. Senator? I would think he knew a thing or two about how to influence politicians.
Anyway, I am a member and contribute to both our national orgs (and, btw to the NRA). I just sent a few bucks to this international one that looks good, too:
I don't want to say how much I have wrapped up in guns and accessories, but it's a lot. Donating hundreds each year is a minor detail to what I spend shooting and hunting. I don't mind spreading it around a little bit.