I am using gitlab and mattermost behind reverse proxy on same domain and same server (as I use omnibus installation).
When I try to make the link between mattermost and gitlab by creating a team on the mattermost startup screen, I am redirected to Gitlab to validate the application but I have this error : The redirect URI included is not valid.
I am not really sure because I got it work but I did not liked Mattermost, I move to RocketChat.
I thought that Mattermost would be integrated inside GITLAB but it is not like this.