GitLab's multi-project pipelines trigger downstream jobs, but you're still testing each MR against the current merged state of dependencies.
Zuul's whole thing is testing unmerged changes together.
You've got MR A in repo 1, MR B in repo 2 that needs A, and MR C in repo 3 that needs B... all unmerged. Zuul lets you declare these dependencies and tests A+B+C as a unit before anything merges. Plus it speculatively applies queued changes so you're not serialising the whole lot.
GitLab has the mechanism to connect repos, but not the workflow for testing a DAG of unmerged interdependent changes. You'd need to manually coordinate checking out specific MR branches together, which is exactly the faff Zuul sorts out.
GitLab's multi-project pipelines trigger downstream jobs, but you're still testing each MR against the current merged state of dependencies.
Zuul's whole thing is testing unmerged changes together.
You've got MR A in repo 1, MR B in repo 2 that needs A, and MR C in repo 3 that needs B... all unmerged. Zuul lets you declare these dependencies and tests A+B+C as a unit before anything merges. Plus it speculatively applies queued changes so you're not serialising the whole lot.
GitLab has the mechanism to connect repos, but not the workflow for testing a DAG of unmerged interdependent changes. You'd need to manually coordinate checking out specific MR branches together, which is exactly the faff Zuul sorts out.