The master node is on-prem? Does that mean that this requires that Google has permanent access to my intranet ?
The entire cluster is on-prem. At the moment, you can optionally leverage a secured tether to manage your cluster in GCP with the same management features you've come to expect with GKE proper. If the connection is lost, your cluster is still fully functional, so no there is no requirement for permanent access to your intranet. The access, when it exists, is also secured to only permit specific access between Google's network and your cluster.