It is possible to copy the pull request to Gerrit easily: paste the contents of https://github.com/wikimedia/mediawiki-core/pull/19.patch into https://tools.wmflabs.org/gerrit-patch-uploader/ and bonk the button. This is of course a one-time, one-way synchronisation.
YuviPanda used to have a bot that provided two-way synchronisation between Gerrit changesets and GitHub pull requests, but I haven't seen it used recently and I don't know where to find the source code or anything.