The best replacement for gerrit is github.

The best alternatives to gerrit are rated as following:
★★★★★  github
★★★★★  crucible
★★★★  phabricator
★★★★  stash
★★★★  gitlab
★★★  rietveld
★★★  gitolite
★★★  github code review
★★★  barkeep
★★  code collaborator
★★  gitorious
★  github pull requests
  pull requests

Your alternative for gerrit is not listed? Suggest!
Speaking about gerrit, one probably shouldn't forget about code review.

Other search terms related to gerrit and their ratings are:
★★★★★ code review
★★★ gitolite
You have in mind a companion for gerrit and it is not yet here? Propose!



Google Trends

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gerrit
github
crucible