I have the exact same question, and 4 years after this question was asked the answer is still somewhat fuzzy. More than that, some people are downright disrespectful when you insist on an appropriate answer (see How to enable chat by @LonelyPixel).
Can anyone point to a document that explains how to populate the User Settings > Chat page?
EDIT: I contacted GitLab support. There is now a GitLab issue on that: #340480