If you add
/api/v4/version to the end of the URL for your GitLab Self-Managed instance the resulting page will show you your version information.
For example, our URL is
http://gitlab.codestream.us/api/v4/version, which results in:
version: "13.6.4"revision: "2754dab2b1b"
If you'd like to use CodeStream's integration with merge requests from GitLab Self-managed, you'll need version 12.10 or later.