Here are a few scripts that use skopeo to copy all tags from one registry to another. It also supports getting a list of "sub-registries" from GitLab and copy all of them. As well as copying Helm charts from GitLab Helm repo to OCI.
https://gist.github.com/StianOvrevage/c5f7d0783edf6aa84494cfdcde5ac5b4