Earlier quoted context omitted.
BuildKit looks nice and their documentation is really nicely structured. Is there anything they have that you're missing in GitLab?
BuildKite is really nice, they also support pipelining and are acting as glue between CI and Github. I can't answer your question about GitLab since I haven't used your product. A plus for us was also that they are willing to support Gogs if we need it.
In the upcoming release (this Friday is the 22nd!) we'll likely ship a pipeline view in the UI https://gitlab.com/gitlab-org/gitlab-ce/issues/3743
GitLab CI is part of GitLab itself and won't support other systems. If you want you can mirror repositories on GitLab EE/.com with http://doc.gitlab.com/ee/workflow/repository_mirroring.html