A merge request is a series of bundled commits layered in a web database somewhere to break compatibility with vanilla Git workflows. That's why Gerrit is better if federation/portability is valuable. 1 commit = 1 code review.

You can push a commit to Gerrit for the webui, you can email it to someone for manual code review and merging, you can (maybe) do it in Phabricator, so on and so forth.

This is inherently federated in a way that GitHub's pull request system does not. A commit in GitHub requires context in the form of a "pull request" provided by their proprietary interface and so the raw git commit doesn't have everything you need to perform the review.

Apologies for not communicating this better in my original comment.