Customize the complete sign-in page

Please see Branded login page

Add a welcome message to the sign-in page (GitLab Community Edition)

It is possible to add a markdown-formatted welcome message to your GitLab sign-in page. Users of GitLab Enterprise Edition should use the branded login page feature instead.

The welcome message (extra_sign_in_text) can now be set/changed in the Admin UI.
Admin area > Settings