While I find it unfortunate that GitLab doesn't have it's enterprise edition open sourced, I think they have enough goodwill in the community so that I'm not that angry about it. I would really be interested in hearing from the GitLab folk whether they think making their enterprise edition open source would really hurt their income and if they have any plans on eventually releasing their enterprise edition as open source (again).
TLDR; GitLab has an open core business model.