Hi - with GitHub everything is configured automatically. A future feature may implement this for BitBucket as well, but you can get a similar experience today by manually configuring the component parts (mirroring, webhook to GitLab on commit, add a script to push the pipeline status to Bitbucket). There is a guide here on how to do that: https://docs.gitlab.com/ee/ci/ci_cd_for_external_repos/bitbu...

Bitbucket Cloud PM here. We'd love to see more usage of Bitbucket (both Cloud and Server) by GitLab users.

As williamchia mentioned above, it's possible to configure GitLab builds for a Bitbucket Cloud or Server repository via webhooks and our build status API today. We're open to helping build better integration, but given the APIs already exist, it would mainly be a project on the GitLab side.