alt Hacker NewsI would use RFC2822 as the underlying format to store any kind of message (pull request, review comment, issue etc.), and of course when displaying messages use CommonMark to style them. Any metadata goes into headers, and Message-ID/In-Reply-To/References headers can be used to link them all together. Using this well defined format you can then decide how to best store and transport all the messages, maybe in a git repo as well, use email, or whatever else works.
I personally think Gerrit works much better than whatever GitHub et al. have for code reviews. As for CI, I would try to keep that out of it as much as possible; just hooks to start a pipeline and to display the result and decide whether to allow a merge or not.
So I think part of the attraction with GitHub is the integration of all 4:
1. Code review 2. Source browsing 3. Ticket tracking 4. CI
It does a mediocre job at all 4. But it does a good job of integrating them all together.
So I agree Gerrit is the superior code review model. but without the other 3 pieces you don't have a product. Even when I was at Google and working every day in Gerrit, I was dissatisfied with the poor integration between code search and code review and with CI.
Google3/Critique/Forge/etc -- Google's internal tooling -- did a much better job of tying that all together.