That is correct. gsutil will be a one-time thing. If you want to ensure, constant replication with metadata tracking with parallel workers for transfer(fast), this will be good. And I think they support have AWS, Azure and Ceph along with GCP
Zenko supports replication from Scality's RING, AWS S3, and other S3-compatible hosts (Wasabi, CEPH, and Scality S3 Connector) to AWS-compatible (Wasabi, DigitalOcean, RADOS Gateway), Azure, and GCP target clouds.
gsutil sync is not slow (worked at Google for years and transferred petabytes quickly with it). what does that mean not resuming transfers? It picks up where you last ran. Do you mean for an individual file?