If your news server offers groups that are really mailing lists gatewayed to the NNTP server, you can read those groups without problems, but you can't post/followup to them without some difficulty. One solution is to add a to-address to the group parameters (see Group Parameters). An easier thing to do is set the gnus-mailing-list-groups to a regexp that matches the groups that really are mailing lists. Then, at least, followups to the mailing lists will work most of the time. Posting to these groups (a) is still a pain, though.