From a VM in GCE, I did the following
gcloud auth activate-service-account --key-file <blah>
# "blah" is a service account key file (JSON) I generated from the web interface
gcloud config set project <project-name>
gcloud config set compute/zone <zone-name>
gcloud set container/cluster <cluster-name>
Then when I tried to run
gcloud container clusters get-credentials <cluster-name>
and it failed with the error message:
Error message: "ERROR: (gcloud.container.clusters.get-credentials) ResponseError: code=403, message=Request had insufficient authentication scopes."
The VM is on the same network as the GKE cluster. I tried the same thing, with the same service account key file from a machine outside GCE, against a GKE cluster on the "default" network and it succeeded...