Is the error happening on GitLab.com SaaS, or is this instance maybe the self-managed GitLab instance on gnome.org? If the latter is true, I suggest opening a thread on the Gnome forum in https://discourse.gnome.org/
For others who find their way here, the issue on gitlab.gnome.org is that you can’t fork or create any projects until you have configured glitlab.gnome.org with an SSH key.