Skia (https://skia.googlesource.com/skia.git) is mirrored in Github (https://github.com/google/skia). Skia sometimes get PRs in Github (example) and then have to ask developers to re-upload in Gerrit.
Skia uses Copybara to automatically create a Gerrit change from a Github PR (after CLA is signed). The Gerrit change created by Copybara is very hard to find (screenshot). The codereview-watcher framework attempts to fix that by updating the PR with a descriptive message.
Design doc is here.