Review Board allows you to browse commits and post them for review on many hosting services. This was once supported on GitLab for public and private repositories, but has not worked since the GitLab 11.3.11, 11.4.8, and 11.5.1 security releases. These disabled access to the URL for fetching a commit's diff via a Personal Access token when the repository is private.
Modern versions of GitLab do include an explicit API for fetching a commit's diff, but the resulting diff is incomplete, lacking crucial information required for fetching the content the diff applies to. It can also result in a truncated or corrupted diff. See our GitLab bug report for more information.
Due to this, we unfortunately cannot post changes for review on modern versions of GitLab via the New Review Request page. We instead recommend that people use RBTools to post changes (see our guide on the RBTools Git workflow to get started).