Discussions about Gnus development takes place on the ding mailing list. It's a mid-volume list (10-30 messages per day) and is open for anyone interested. To subscribe, send a message to [email protected] to subscribe. The address of the mailing list itself if [email protected]. This list and the others are gatewayed to GMANE, which includes a searchable index of messages.
If you aren't interested in discussions, but just want to keep up-to-date on new Gnus releases (of which there is (on average) one every two days), you can subscribe to the announcements list. Send a mail to [email protected] with the text "subscribe ding-announce" in the body of the mail to subscribe.
If you want to have an up-to-the-second version of Gnus available, but don't want to have to pull down a new version manually, you can subscribe to the patch list. You'll then receive the patches for each release in the mail. Send a mail to [email protected] with "subscribe ding-patches" in the body of the mail to subscribe.
On Usenet, gnu.emacs.gnus is the appropriate place to ask questions on usage of released versions of Gnus.
The mailing lists are gatewayed to GMANE, which lets you read via NNTP (in Gnus, for instance) or your web browser.