GitHub has most of the Gerrit repos at https://github.com/wikimedia , making it very convinient to experiment with the code by simply forking it right there on github and experimenting with it. For example, mediawiki itself has close to 900 forks. Yet, some repos like https://phabricator.wikimedia.org/source/wikibase-serialization/ (possibly just those in Phabricator), do not appear to be there.