TLDR; Why GitLab instead of GitHub? From their discussion group thread:

- Good multi-platform hosted CI or at least workable integration with other (existing) CI solutions

- Hosted review tool that we don't have to maintain ourselves (though a little bit less good than Phabricator, allegedly)

- familiar GitHub-like workflow with no requirement to install extra software locally

- Reuse of GitHub credentials

- Realistic path forward for migrating tickets from Trac

CI is becoming Achilles heels for GitHub. Personally I would probably also add lack of fine grained permissions for contributors, inflexible diff, very basic code review tool as well.