GitLab CI
Also, do you know if it's possible to use gitlab CI as a merge gate on github PRs?
To answer your questions: yes, you can configure the Runner to work behind a firewall [1] and you can use GitLab CI for GitHub repos [2].
Hope that helps!
1 - https://forum.gitlab.com/t/how-does-communicate-gitlab-runne... 2 - https://docs.gitlab.com/ee/ci/ci_cd_for_external_repos/#pipe...