Mattermost 4.0.2 has been tested with GitLab SSO with official release on July 31 from https://about.mattermost.com/download/.
The patch release fixes the following issues. Upgrading is recommended.
- Fixed a login issue when a non-English language is set in Mattermost Account Settings. Affects accounts with GitLab single sign-on.
- Custom slash commands now work in direct message channels.
- GitLab single sign-on in Mattermost mobile apps no longer redirect to a browser page.
Mattermost team is ready to test GitLab 9.4.X omnibus RC for both upgrade and new install.
Please let me know if you have any questions. Thanks!