Suppose a GKE cluster resides in GKE_PROJECT_ID different from HUB_PROJECT_ID. In such a multi-project registration scenario, the command line param --gke-cluster won't work. I believe we'd have to use the --gke-uri param instead.

BTW, --project in this command refers to the HUB_PROJECT_ID. When GKE_PROJECT_ID == HUB_PROJECT_ID, this distinction isn't important. But in a multi-project registration scenario, it is better to rename ${PROJECT_ID} to ${HUB_PROJECT_ID} in order to be more clear on the concept.