We are offering our top tiers (Ultimate or Gold) for free to open source projects [1]. You can see the complete list in our repo [2]. Some of the most recent projects that received our free license are Arch Linux, the Tor Project, edX (currently migrating). Just to add a few more: Drupal, Haskell, VLC, Kali Linux, Lineage OS, and more on this comment [3].
[1] - https://about.gitlab.com/solutions/open-source/program/
[2] - https://gitlab.com/gitlab-com/marketing/community-relations/...
[3] - https://news.ycombinator.com/item?id=18724015