I've been working on a bot to copy Github Pull Requests into Gerrit reviews, using the Google-hosted Gerrit servers.
My bot isn't running publicly yet, but you can see the start of it working at:
https://github.com/grpc/grpc-go/pull/546 or https://github.com/grpc/grpc-go/pull/545
(Code is at https://github.com/LetsUseGerrit/gerritbot for now, but I don't imagine anybody would use the code themselves... they'd only interact with it via the Github bot)