I'm aware. But the important features for a small company (external auth, ldap) are on a paid tier for Mattermost. Its good that mattermost can tie into gitlab's authentication via ldap (no ldap group permissions though with the OSS tier).
Edit: Gitlab doesn't officially support SSO on top of SSO (AD/LDAP, etc), so its at your own peril: https://docs.mattermost.com/deployment/sso-gitlab.html