Disclosure: I'm the CEO of GitLab and we compete with GitHub.
We think of ourselves as an open source company. But today paxcoder on Hacker News rightly remarked that calling it an open core company is more accurate.
We ship GitLab CE which is open source and GitLab EE that is closed source. We try to be a good steward of the open source project. GitLab EE is proprietary, closed source code but we try to work in a way similar to GitLab CE: the issue tracker is publicly viewable and the EE license allows modifications.
When we mention that GitLab is available in an open source edition people frequently ask "Isn't GitHub open source?". I understand the confusion between open source hosting and open source software. The hosted service GitHub.com is free for open source projects and it has fundamentally improved open source collaboration. But the software GitHub's service is based on is closed source.