When your book is created on gitbook.io, you need to push some content to it.
If you don't have the editor yet, install it for free.
After connecting your GitBook.io Account to the editor.
You can use the menu Book > Publish to update your book content.
You can use Git to push update of your book.
$ git push https://push.gitbook.io/{{UserName}}/{{Book}}.git
It will prompt for your username (username or email) and your password (password or api token).
Once you pushed content to GitBook.io, you need to check the state of your build.