Broadly speaking, I don't know why software is singled out in its economics. Creating software takes time of people. Time costs money, and much more of that when it comes to highly skilled engineers. The fact software has virtually no replication costs does not change the fact that its engineering is as expensive if not more so than engineering of other goods.
GitLab is open core, not open source: "We ship GitLab CE which is open source and GitLab EE that is closed source." from https://about.gitlab.com/blog/2016/07/20/gitlab-is-open-core...
(As an unrelated aside, given the number of people on HN that mistakenly think GitLab is open source, I'd say that "open core" as a marketing tactic has a real chance of becoming widespread in the future.)