My org uses mattermost - a logical addition since we were already heavily invested in gitlab. I don't run the services personally but I know the guys who do, a

My org uses mattermost - a logical addition since we were already heavily invested in gitlab. I don't run the services personally but I know the guys who do, and from what I can tell it mostly Just Works. We use our org's SSO for gitlab, and we use use gitlab auth for mattermost, and all the pieces work together well. I don't use IRC with mattermost, but there are bridges available.