How GitLab is open source? It open sources a non-essential portion of its software, and so does GitHub.
Calling what it open sources "non-essential" is so ridiculously misinformed that I'm really wondering what your goal is here.
But EE, the product they are making money from, is not free, and not open source. Thus GitLab is not "Open Source company", as stated by arnieswap at the beginning of this thread.
I don't think this is a fair (or accurate) quantification.
While Gitlab's principal product probably has more open sourced code, Github makes tremendous contributions to open source. Atom is open source. They contribute significantly to projects like Rails (for example, the multiple database feature was extracted from their codebase). This doesn't even begin to address the dependency upon Github the entire open source ecosystem has. How many millions of dollars do they spend in infrastructure every month just to keep projects like RubyGems and npm viable?
I think Gitlab is a spectacular offering, and having a single source for your entire dev-ops story is extremely compelling. However, spinning a narrative that Gitlab is more open source than Github is extremely disingenuous.