In doc https://docs.openshift.com/container-platform/4.16/machine_management/control_plane_machine_management/cpmso_provider_configurations/cpmso-config-options-gcp.html#cpmso-yaml-provider-spec-gcp_cpmso-config-options-gcp 

obtain the path to the image by running
$ oc -n openshift-machine-api \
  -o jsonpath='{.spec.template.machines_v1beta1_machine_openshift_io.spec.providerSpec.value.disks[0].image}{"\n"}' \
  get ControlPlaneMachineSet/cluster

but this part we said "When you create a control plane machine set for an existing cluster", we are creating a ControlPlaneMachineSet, so ControlPlaneMachineSet is not exist in cluster, we can get from machineset

$ oc get machinesets.machine.openshift.io -n openshift-machine-api   
NAME                        DESIRED   CURRENT   READY   AVAILABLE   AGE
oadp-92591-q6lgb-worker-a   1         1         1       1           4h42m
oadp-92591-q6lgb-worker-b   1         1         1       1           4h42m
oadp-92591-q6lgb-worker-c   1         1         1       1           4h42m
oadp-92591-q6lgb-worker-f   0         0                             4h42m

$ oc -n openshift-machine-api -o jsonpath='{.spec.template.spec.providerSpec.value.disks[0].image}{"\n"}' get machinesets.machine.openshift.io oadp-92591-q6lgb-worker-a
projects/rhcos-cloud/global/images/rhcos-416-94-202406251923-0-gcp-x86-64