I’ve searched around and haven’t found anyone who reported the same issue.
In a nutshell, I upgraded Gitlab to 8.2.1 (Omnibus install) and enable Mattermost, set url. I hit the url and see the following in on the page:
All team communications in one place,
searchable and accessible from anywhere.
I do not see the login box or anything else on the page other than the lower third Mattermost banner and links.
I also see this in the logs:
[CRIT] failed to ping db err:dial unix /var/opt/gitlab/postgresql/.s.PGSQL.5432: no such file or directory
Gitlab is working fine.