Do you need GitLab Premium to use this?

In GitLab documentation I noticed, that you need GitLab Premium to mirror repositories from external sources to GitLab. Does Subgit also need GitLab Premium to work?
I use GitLab Enterprise Edition [15.3.1-ee]

Hello!

Thank you for your interest in our product!

Basically, SubGit mirroring feature does not relate in no way to GitLab’s repositories mirroring: GitLab allows establishing mirror between two Git repositories while SubGit mirrors SVN and Git repositories. Also, SubGit is an external tool completely independent of GitLab and it has no requirements about GitLab edition or license supporting any edition starting from CE. SubGit is able to work with GitLab projects mirroring them to SVN, but it does it as external tool running as a separate process on a GitLab server and reading and writing GitLab repositories directly.
Just in case, here is our documentation article describing using SubGit with GitLab:

TMate SubGit: SubGit for GitLab

Additionally, note, that SubGit requires direct filesystem access to the repository files and it uses hooks to intercept push or merge operations, so if you intend to run GitLab in a container, consider embedding SubGit inside that container, too, as running SubGit in a side-car container will prevent hooks from working thus making mirror half-functional.