How to explain GitLab to anyone

How do you explain what GitLab is and how it works to a non-technical person? Here's a real-world analogy to help.

WIP Google doc: https://docs.google.com/a/gitlab.com/document/d/1nRYPDr1I9CrIh9qjhtfZDRYRkSyV1MuJ0Sx2itcG69E/edit?usp=sharing