We recently conducted an extensive evaluation of CI options[0], concluding at the time that GitLab could meet our needs but didn't make a whole lot of practical sense unless we also migrated to it for code review. Other considerations (Gerrit user experience and sustainability, onboarding costs, a de facto migration of many projects from our Gerrit instance to GitHub, etc.) led us to re-evaluate whether a migration for code review would make sense, and that's what the decision linked here addresses.
I am on the team that maintains our existing CI system[1] (Zuul, Jenkins, JJB), though I mostly work on other things. While this system is certainly quite powerful, I would not personally describe it as easy. We have a lot of work in front of us in migrating it to GitLab, but so far I've found the experience there quite a bit more pleasant than grepping through JJB definitions and the like.
At any rate, if you're interested in how all of this pans out, we will as ever be doing the work in a very public fashion.
[0]. https://www.mediawiki.org/wiki/Wikimedia_Release_Engineering...