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.
--
Matma Rex