you can look at the source, but it is not "open source": https://gitlab.com/gitlab-org/gitlab-ee/blob/master/LICENSE
Isn't this the vision that Stallman had about open source or free software and copy left?
Free Software according to Stallman's vision includes the right to distribute your modifications. You are not allowed to do that with GitLab EE.
This is generally called source available or public source, like the Unreal Engine, rather than open source, which implies you can actually do something with that source code.
[1] https://en.wikipedia.org/wiki/The_Open_Source_Definition