Is it possible to allow users to sign up on a GitLab instance and only give them the right to fork existing repos but not to create new ones?
As per the docs: Permissions and roles | GitLab
probably not, at least some minimalisation is possible with a Gitlab subscription, but probably not the complete customisation that you would like. Also from a post on this forum already: Is there any way to customize user role permissions on gitlab.com? there was an issue open for custom roles, but doesn’t seem to be open where gitlab-foss was migrated to: Issues · GitLab.org / GitLab · GitLab
Maybe you can search on that issue list and open an issue for such custom role/permission ability, but I expect even if it does become available, it will only be in Premium or higher, which requires a paid subscription.
Thank you very much for your reply and the time you spent lookin for the related resources.