Where GitLab may have an edge is that any open source project inevitably results in a version that's perpetually free to use. GitHub Enterprise has a 45-day free trial, but open source comes with an implicit guarantee: What you see is exactly what you get. Modern developers are conscious of -- and highly allergic to and deeply vocal about -- the limits of free-to-use-but-not-open-source products, so GitLab will have some leverage.
Which is stronger: the open source allure of GitLab or the familiarity of GitHub? Rank-and-file developers who are already comfortable with GitHub -- including GitHub's quasi-proprietary feature set -- may decide GitHub and its associated technologies are open enough. The lack of an open source version of GitHub may be no an obstacle for them, and GitLab being open source may not prove enough of an attraction.
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