Creating regular Persistent Disk Snapshots to backup data, even if they’re attached to a running instances. Snapshots are a faster, more cost-effective way to back up data versus to create a full image of a disk.

Snapshots are differential, so administrator can create regular snapshots on a persistent disk much faster and at a much lower cost. Snapshots are always created based on the last successful snapshot taken.

use the gcloud Compute Disks Snapshot command:

gcloud compute disks snapshot [DISK_NAME]

Reference
Creating Persistent Disk Snapshots