@mrcaron, @gl893, @itreers, @jweare, @rompic, @zwarag, @jirka
Just chiming in on this thread, if you want GitLab and Mattermost to work out-of-the-box, just put the Mattermost webhook URL in GitLab's Slack integration, per the documentation.
If you want to do custom transformations with how to render GitLab's raw webhook info into a Mattermost post (e.g. add custom emoji, make a table, etc.) there's an open source integration service to set that up.
Sorry, when we redesigned the UI for webhooks we somehow lost the links to documentation that used to be in the product.
It shouldn't be this hard.