While gitbook does wrap the branching/merging process in a UI that is digestible by non-developers, Retype is far more powerful because you actually use GitHub (or GitLab). You have the full power of GitHub, including branching, pull-requests, issues, reviews, automation, authentication, and everything else.
For example, here's the repo for the retype.com website, see https://github.com/retypeapp/retype
The entire website is built from simple Markdown text files.