I designed a fairly complex test matrix with a lot of logic offloading to the control mechanisms Gitlab offers. You create job templates or base jobs that control the overall logic and extend them for each particular use case. I had varying degrees of success, and it's not a job for a Devs side quest, that means I think you need someone dedicated to explore, build and debug these pipelines, but for a CI tool it's very good.

Because you can extend and override jobs, you can create seams so that each piece of the pipeline is isolated and testable. This way there is very little that can go wrong in production that's the CI fault. And I think that's only possible because of the way that Gitlab models their jobs and stages.