Getting Mattermost working with GitLab 6.8.1 3197990

We are on GitLab 6.8.1 and would like to use Mattermost. We use LDAP/AD to login to GitLab and would like to use same for Mattermost as well.

What is the best way forward for us? Should we install Mattermost separately? if so, can we still use LDAP/AD to login to it? Should we upgrade our version of GitLab?

Your best bet is to upgrade GitLab yes. Mattermost uses oauth to authenticate so any registered GitLab user will be able to authenticate. See https://github.com/mattermost/platform/blob/master/doc/integrations/Single-Sign-On/Gitlab.md