This one work fine, assuming you are in the correct region/zone (use gcloud config set compute/zone <my-zone>):

gcloud compute disks delete $(gcloud compute disks list --filter="-users:*" --format "value(name)")

You can use the gcloud compute disks delete command in cloud shell to delete all the disks that are not attached in a gcp vm instance.

gcloud compute disks delete DISK_NAME DISK_NAME … GCLOUD_WIDE_FLAG …

you can provide disknames through this command to delete them.

disk delete

The link that @sandeep-mohanty includes suggests that only non-attached disks are deleted by the command.

Assuming (!) that to be true (check before you delete), you can enumerate a project's disks and then delete the (not attached) disks with:

PROJECT=[[YOUR-PROJECT]]

# Get PAIRS (NAME,ZONE) for all disk in ${PROJECT}
# Using CSV  (e.g. my-disk,my-zone) enables IFS parsing (below)
PAIRS=$(\
  gcloud compute disks list \
  --project=${PROJECT} \
  --format="csv[no-heading](name,zone.scope())")

# Iterate over the PAIRS
for PAIR in ${PAIRS}
do
  # Extract values of NAME,ZONE from PAIR
  IFS=, read NAME ZONE <<< ${PAIR}
  # Describe
  printf "Attempting to delete disk: %s [%s]\n" ${NAME} ${ZONE}
  # Deleting a disks should only succeed if not attached
  gcloud compute disks delete ${NAME} \
  --zone=${ZONE} \
  --project=${PROJECT} \
  --quiet
done

NOTE In the unlikely event that Google changes the semantics of gcloud compute disks delete to delete attached disks, this script will delete every disk in the project.