You're mistaken: The source code is available, but you're not allowed to use it without a valid subscription [1]! Only the GitLab Community Edition (CE) is freely usable without a license, the Enterprise Edition (EE) always requires a subscription.
To be honest I find the licensing situation more than confusing (even if the GitLab team continuously claims to be invested in Open Source) and I'm not too fond of it.
[1] https://gitlab.com/gitlab-org/gitlab-ee/blob/master/LICENSE