Hi, I'm in (was in?) GameJournoPros. The topic has been discussed to death, with both myself and others chiming in about it.
Here's one of the main things you should know: "Press Clubs" have existed for well over a hundred years.
The Denver Press Club claims to be the first one in America, having been founded in 1877. But if you Google "[Major city near you] press club," you'll almost certainly find one that many of your local newspaper and TV journalists are part of.
Maybe GameJournoPros should have been more public about its existence in the same way that those press clubs are (though it wasn't some huge secret. You know how I found out about it back in 2012? Twitter), but that's all it is. A press club. It's a collection of people in the same industry talking about issues related to that industry. Issues like adblock, embargoes, dealing with stingy PR...
Journalists know each other. Sometimes a coworker/friend at one outlet will move to another outlet. Sometimes people just get to know each other by covering the same events at the same times. There are no journalistic ethics rules against this.
Furthermore, press clubs are neither illegal or unethical. My bosses knew about GameJournoPros and that I was in it. They have no problem with it.
You could say that there's a worry about such a group being or becoming an echo-chamber, and I actually do think that's a fair concern. But here's the thing with writers: Most of them tend to disagree with each other. A lot. You could see some of that in the e-mails Breitbart has leaked, but it's been even more heated than that. Heck, we can't even agree on if review scores are a good thing or not.
If you want to find corruption in the games press, go ahead. Scrutinize ad deals, question free gifts/travel, make noise if a site seems to bow to pressure from advertisers (like in the oft-cited Jeff Gerstmann example). But a mailing list full of journalists isn't the corruption you're hoping to find.