We have observed that GKE does not delete the Disks even after deleting the cluster which results into lots of Disks for the cluster which don't even exists.
In below link, It is mentioned that GKE will not delete persistent disks but what is the rational behind not deleting the disks.
https://cloud.google.com/kubernetes-engine/docs/how-to/deleting-a-cluster#overview
If we are deleting the cluster it means we don't need the resources associated with it so ideally it should delete disks too which would help in saving the storage cost too.