GitLab team member here! User Limits will not apply to a public group as a top namespace and as at this time, no limit on private groups within a public top namespace group. See the 6th Q&A entry in the FAQ page (https://about.gitlab.com/pricing/faq-efficient-free-tier/#us...), excerpt here:

Q. Do these changes apply to private projects within a top-level namespace with public visibility? A. User limits are currently applied based on the visibility of the top-level namespace. We will monitor how top-level namespaces with public visibility are using private projects to identify whether any limits on such projects are needed.