As I'm probably the steward on this going forward: unknown. I don't intend to implement them unless I need them. Would I take a patch with good coverage that implemented those.
Fwiw, the GCS client for go (import "cloud.google.com/go/storage") is very straightforward. Though as others have pointed out, it might be worthwhile to just try to use minio-go if you want to gain Ceph as well.
Disclosure: I work on Google Cloud (and if the way is paved, we will contribute here; seems like a great project)