A couple questions:

Your website talks about GitHub. Is it specific to GitHub or does it work with any Git repository?

What if you shut down? How do I continue to produce my book/paper?

We have great GitHub integration because it's the main Git host out there, we store git repos too.

If we shutdown, our toolchain and book format is already open source (https://github.com/GitbookIO/gitbook), so no matter what you'll always be able to use that to build your book locally. Secondly our Desktop Editor (https://www.gitbook.com/editor) works offline so you could even continue to use that. And well since your book is a git repo, you can have a local copy, a copy on GitHub or any other git server of your choice. So I think you should be good :)

We don't believe in vendor lock, we're big supporters of open-source and we want users to use our tools and platform not because they're obliged to, but simply because it's the best workflow/product for them.

Does that make sense ?