Back when it started, it was great. Ray Reynolds and another guy started it together. The other guys' name slips my mind, but he was an attorney and set it up as a legally-organized and registered Association.
I can assure you when it was new, it had some really useful information and the newsletter was really good with lots of information.
Ray was the kind of collector who wants every variation of a particular model. He had something like 30-40 Glock 21s - one for every engineering change that came along. Not sure if he is still collecting or still involved with the association or not. It's been a LONG time since I've heard from him or spoken with him. Ray was a regional sales manager for Glock for like the first 10-15 years Glock was in the US.
At the last NRA show I went to, in Louisville, KY (2008?), the GCA had a booth and three guys in it. They're still around and still taking applications. They have some really collectible Glocks (saw a prototype first-gen G-19 made from a cut-down G-17) and some really good knowledge base. They were able to confirm for me that the Ducks Unlimited Commemorative was a one-of-a-kind, even though it had like serial number 218...
It would be nice if someone or some group could combine the GCA, this Collector's Forum, the Glock Useful Information Forum (at least the serial number research thread), and GlockFAQ.com and put it all together under one organization. That would be a knowledge base with a web presence, and they could do the newsletter electronically, which would save on costs, and there'd be enough interest to attract good contributions from members. It'd be a great knowledge base, all in one place.