Hello! So I tried converting https://github.com/wikimedia/qa-browsertests/pull/1 into a Gerrit changeset (https://gerrit.wikimedia.org/r/#/c/54097/) , and was mostly successful. It is also a relatively painless process - at least for single commits.
This assumes you (person doing the GitHub -> Gerrit bridge) have a Gerrit account. I wrote a small script that sortof makes this easy:
https://gist.github.com/yuvipanda/5174162
This only does things one time - it moves a set of commits in a pull request to a squashed single commit on gerrit, assuming your current directory is a cloned version of the gerrit repo you want to commit to. It should not to be too hard to write an actual, idempotent sync script that maintains a 1-to-1 correspondence between Pull Requests and Gerrit Changesets, and I'll attempt to do that tomorrow.
Note that this is a shitty bash script (to put it mildly) - but that seems to be all I can write at 5:30 AM :) I'll probably rewrite it to be a proper python one soon. That should also allow me to use the GitHub API to also mirror the GitHub Pull Request Title / Description to Gerrit.
I also offer to manually sync pull requests into gerrit as they come until the automatic Gerrit integration is ready. Shall be writing another small script tomorrow to have me 'watch' all the wikimedia/* GitHub repositories.
Thank you :) I'll update this thread as the script gets less shitty. Do let me know if you have build a far more complete script :)