Hi!
This is an irreversible process. There’s no way to restore a VM or PD without a Snapshot. Snapshot creation is customer's responsibility and is easily configurable for automated snapshot creation for VMs. [3]
As per our public documentation “Deleting a disk removes its data permanently and is irreversible. However, deleting a disk does not delete any snapshots previously made from the disk. You must separately delete snapshots.” [1]
And public documentation for Persistent disk encryption “When you delete a persistent disk, Google discards the cipher keys, rendering the data irretrievable. This process is irreversible.” [2]
It is recommended that any data that you would consider to be important should have regular snapshots scheduled. This document will help you to implement automated snapshots for your VMs[3][4]. It is also recommended that you configure your instances so that they are not accidentally deleted in the future[5].
This document contains details on how you can design a more robust system[6].