We're working on shipping a high quality set of APIs. We're doing these in parts so we can design and document them well.
Mattermost v1.1 ships on October 16 and offers incoming webhooks as a start: http://www.mattermost.org/webhooks/
You'll find a link there to an open source sample app for customizing how you want webhook events from GitLab to appear.
Mattermost incoming webhook support ships with GitLab 8.1 on October 22.
For Mattermost v1.2 (November 16), we're working on outgoing webhooks, which could offer the ability to fire off console commands.