Cloud Storage uses a flat namespace to store objects in buckets. While you can use slashes ("/") in object names to make it appear as if objects are in a hierarchical structure, the gsutil stat command treats a trailing slash as part of the object name.
For example, if you run the command gsutil -q stat gs://my-bucket/my-object/, gsutil looks up information about the object my-object/ (with a trailing slash), as opposed to operating on objects nested under my-bucket/my-object/. Unless you actually have an object with that name, the operation fails.
For subdirectory listing, use the gsutil ls instead.