No I'm pretty sure that's just a sort of 'integration', Mattermost shipped with GitLab?

https://about.gitlab.com/blog/2015/08/18/gitlab-loves-matter...

> Like many companies in the last year we've switched to using Slack to improve internal communication. [...] Since Slack doesn't offer an on-premises version, we searched for other options. We found Mattermost to be the leading open source Slack-alternative and suggested a collaboration to the Mattermost team.

I'm not really sure why it's 'GitLab Mattermost' and not (at your link) 'GitLab Nginx' et al. though.