What is "Gitlab Mattermost"?

I’m try to find what is exactly “Gitlab Mattermosr” and how can I use it out of the Omnibus package. I spend already long time to search in the doc.

Is it a standard Mattermost installation with auto-configuration to be able to use SSO of Gitlab ? If yes so is there an easy way to install Mattermost, and link it to Gitlab ?

Or is it a special installation of Mattermost including some features/modules to integrate it deeply in Gitlab ? And so, is there any easy way to install it without Gitlab, and out of Gitlab Omnibus ?

Got it from http://forum.mattermost.org/t/features-of-gitlab-mattermost/297 : gitlab-mattermost is a modifed Mattermost.

If you install Mattermost alone and connect it to gitlab: you willNOT have the same whatever you do.