@serge
Thanks for posting that. You got further than I did!
I had been trying to engage with the gitlab developers to find a solution here ... but they suggested going back to mattermost developers ... which I tried here (on the forum) and on the developer channel ..
While gitlab/mattermost integration works nicely for many things ... being able to access the mattermost API when authenticating with gitlab is a non-runner, unless you somehow backward-engineer things ... which is certainly frustrating.
I would love if there was some renewed focus placed on solving this.