The new version of GitBook official web service (I called it GitBook V2 for short) has changed its product policy to be profit-oriented and not open source anymore. The important point is GitBook V2 does not offer unlimited books anymore. Since that, I tried to find another solution to publish GitBook.
Mostly I use GitBook to record some notes for sharing and reading. Additional fees or complex operations are not preferred.
In general, I prefer a solution: free, unlimited books, publishing automatically after revising, and easy configuration.