Was Github ruled out because it's not free enough in spirit or are the Gnome folks just wanting to run things on-prem?
As discussed on the article, GitLab also wasn't an option for them before the change from a CLA to a DCO. The CLA meant that although gitlab was licensed under a free software license, GitLab Inc. had permission to relicense future versions of GitLab as proprietary software, without needing to obtain permission from the original contributors. This meant that GitLab used to not be sufficiently free "in spirit" for Debian and GNOME. (While this might seem a bit paranoid, Debian and GNOME want to avoid the worst case scenario of GitLab being acquired by a company that isn't as friendly to software freedom.)
All in all, I think GitLab is doing a great job at working together with free software projects like Debian and GNOME and I am sure all the involved parties will greatly benefit from this collaboration.