Connecting to an IMAP can be very easy. Type B in the group buffer, or (if your primary interest is reading email), say something like:
(setq gnus-select-method '(nnimap "imap.gmail.com"))
You’ll be prompted for a user name and password. If you grow tired of that, then add the following to your ~/.authinfo file:
machine imap.gmail.com login <username> password <password> port imap
That should basically be it for most users.