We did this at my company; we already had self hosted free GitLab. We tied it into our AD and went happily without Hipchat about a year ago. Some things are rougher (image support) but it was mostly smooth and equivalent, and actually nicer in that it supported Markdown.
Recommended. And, a lot cheaper than Slack which is insanely expensive ($72/user/year) - of course it is not free as it takes up AWS server resources and DevOps time, but we control all our data and security now.
Yes, I do. Sorry for the abbreviation. We used the LDAP connector that is publicly available in GitLab and then tied Mattermost into GitLab authentication.
Recommended. And, a lot cheaper than Slack which is insanely expensive ($72/user/year) - of course it is not free as it takes up AWS server resources and DevOps time, but we control all our data and security now.