It depends what you mean by "open." Anyone can subscribe to the go-codereview mailing list. Anyone can submit a patch, but you need to a) use gerrit and b) sign a (digital) contribitors license agreement with Google.
It's "open", but it would almost certainly collapse entirely if Google decided to drop support for it. (There's no sign that they'll do that, but it's not a risk you take with a language like C.)