That's cool. Are they integrated well?

Mattermost can use GitLab for authentication, which is nice. Other than that, you can configure webhooks in GitLab's git repositories to push messages to a Mattermost channel; for example, push every tag created so colleagues can see a new version of one of your libraries/products was released.

There's work in progress with Cog for increasingly powerful Mattermost-GitLab integration: https://forum.mattermost.org/t/help-wanted-cog-integration-f...