So can I create multiple “namespaces” and each one can contain a repository of 5GB?
They need to be top level groups.
But yes, if you have public repos repoa/repoa and repob/repob instead of user/repoa and user/repob, then my understanding is both repoa and repob will hae 5GB each.
If you are not on a paid tier, all your repositories share a pool of 5GB per namespace. A namespace is basically a user or project. So if you don't use namespaces, everything is under your personal user namespace and your limit is 5GB total storage across all your repositories.