Tried out
https://github.com/GoogleCloudPlatform/kubernetes/blob/master/docs/getting-started-guides/coreos/coreos_multinode_cluster.md
with GCE.
It doesn't say how you should connect to your master with kubectl.

For AWS, it appears that one would run kubectl on one's desktop and it would be able to communicate to the master over the VPC because of the VPC rules. There is no equivalent instructions for GCE.

Maybe someone familiar with GCE networking can add some.