Thanks for mentioning GitLab. We would not have been able to grow as fast as we did without adopting an open core model. In the beginning we sold only support but companies tended to cancel their support contract after not needing it for a year. The margins on out self-hosted product are much better than on our SaaS product because of the hosting fees for free users.
An example of a pure free software business model is RedHat, they open source everything they make and that is working out for them https://news.ycombinator.com/item?id=18322772