Gitlab_Mattermost setup unable to redirect http to https

Mattermost and Gitlab are set up in two different servers via omnibus installation. I get a “The redirect uri is not valid” error. Upon further inspection, in gitlab.rb , mattermost external uri is set to '" , the configuration in Gitlab admin dashboard > Admin > Application >Mattermost has https:// URLs. The setup has no mention of allowing HTTP. Redirection of HTTP to https is also enabled.