Do you have any plans to support GitLab, in addition to GitHub?

Not currently. We get a handful of requests per year for it, but currently not enough to justify doing it. Please feel free to contact us with your vote for this feature anytime at hello@leanpub.com, or if you're already an author, on our Authors Forum here: https://community.leanpub.com/t/can-i-deploy-my-ebook-from-a-gitlab-repository/1738.