Some of the folks running this (of which I am one) are going to be talking about it in a livestream in a little over a week: https://openinfra.dev/live/
Zuul is an open-source CI/CD/project-gating-system that implements this for Github, GitLab, Gerrit, and pagure. It actually lets you stack dependencies between any of those systems. Just say "Depends-On: <url>" in the PR.