Those settings correspond to the following fields in gitlab.rb:
- EnableSMTPAuth is
mattermost['email_smtp_auth'] - SkipServerCertificateVerification is
mattermost['email_skip_server_certificate_verification'] - ConnectionSecurity is
mattermost['email_connection_security']
The corresponding ones for GitLab that you listed sound correct. If there’s any other settings missing from your gitlab.rb, you can see every possible one here.
For the ConnectionSecurity setting not being set properly, that’s very strange. STARTTLS is the correct value for it. Maybe you could try configuring EnableSMTPAuth and SkipServerCertificateVersion, running gitlab-ctl reconfigure, and seeing if ConnectionSecurity is picked up then?