GitLab Enterprise Edition does not have an open source license. Since GitLab.com runs GitLab EE, their two (presumably) primary sources of revenue (EE licenses and GitLab.com paid accounts) come from non-open source software.
But: it's still fair to call GitLab as a company way more Open Source than GitHub. All of their development happens out in the open, the vast majority of their codebase is Open Source, and the source code for GitLab EE is even made available.
They are actively working against open source tooling for software development by developing features only for their closed platform.
Gitlab is a much better example of a company that fully embraces open source.