Prio Notes:

We may contact the team in the WMF responsible for migrating from Gerrit to GitLab, in order to consult and assist us with this investigation.

See T350793: move commons-query.wikimedia.org and query.wikidata.org to kubernetes for a discussion about the microsites deployment pipeline for both QB and QS-GUI

The report can be found here: https://docs.google.com/document/d/1A6dSDuF2uSpbS75HWSenEnF2KcSKKpj-ZRAsX38n-nY/edit

Expanding on one of the comments I added in the report for documentation purposes:

The Gitlab Migration Guide specifies:

"This will create a complete copy of the repository, including all branches and review notes from Gerrit."

("This" being importing the project to Gitlab by URL).

I spent some time looking through projects already ported to Gitlab (see list of repos in miscweb) and I couldn't find evidence that review notes are migrated as well.
For example, see this change on Gerrit and its migrated equivalent on Gitlab. In this case the final commit was migrated, but the previous commits in the Gerrit patch and the review notes for those commits have not been included and are nowhere to be found in the Gitlab migrated project.

It might be worth to at least find out what the Gitlab team's approach to this is. The review data isn't lost, it's still on Gerrit and will probably remain there until the foundation stops using Gerrit altogether which I don't see happening any time soon.