The core of the GitLab software is called the CE. It is distributed under the MIT license, which is a permissive free software license created at the Massachusetts Institute of Technology. You are allowed to modify the software and use it in your creations.
No feature that ever made it to CE will ever be removed, or moved to a closed source version. When GitLab EE was created in 2013, it was, at its core, GitLab CE, but it had additional enterprise features, such as Lightweight Directory Access Protocol (LDAP) groups. Those features are not open source, per se, but can be added to the core version if they are perceived by the company as a core feature. The idea was that companies should also contribute as much as possible to solving problems and creating new features.
In 2016, the GitLab EE product was divided into three tiers: Starter...