For doc https://docs.openshift.com/container-platform/4.16/machine_management/control_plane_machine_management/cpmso_provider_configurations/cpmso-config-options-aws.html#cpmso-yaml-provider-spec-aws_cpmso-config-options-aws 
For part "Sample GCP providerSpec values"
1. we'd better change
spec:
# ...
  template:
# ...
      spec:
        providerSpec:
to 
spec:
# ...
  template:
# ...
    machines_v1beta1_machine_openshift_io:
# ...
      spec:
        providerSpec:

2. "labels: null" should changed to "labels: {}"
I tried to change the default  "labels: {}" to "labels: null", new masters will be created.
$ oc get machines.machine.openshift.io -n openshift-machine-api 
NAME                              PHASE          TYPE            REGION        ZONE            AGE
oadp-92591-q6lgb-master-0         Running        n2-standard-4   us-central1   us-central1-a   5h
oadp-92591-q6lgb-master-1         Running        n2-standard-4   us-central1   us-central1-b   5h
oadp-92591-q6lgb-master-2         Running        n2-standard-4   us-central1   us-central1-c   5h
oadp-92591-q6lgb-master-928xj-0   Provisioning   n2-standard-4   us-central1   us-central1-a   15s

3. remove metadataServiceOptions: {} this is an aws field.

The whole cpms on gcp looks like:
$ oc get controlplanemachineset cluster -n openshift-machine-api -o yaml 
apiVersion: machine.openshift.io/v1
kind: ControlPlaneMachineSet
metadata:
  labels:
    machine.openshift.io/cluster-api-cluster: oadp-92591-q6lgb
  name: cluster
  namespace: openshift-machine-api
spec:
  replicas: 3
  selector:
    matchLabels:
      machine.openshift.io/cluster-api-cluster: oadp-92591-q6lgb
      machine.openshift.io/cluster-api-machine-role: master
      machine.openshift.io/cluster-api-machine-type: master
  state: Active
  strategy:
    type: RollingUpdate
  template:
    machineType: machines_v1beta1_machine_openshift_io
    machines_v1beta1_machine_openshift_io:
      failureDomains:
        gcp:
        - zone: us-central1-a
        - zone: us-central1-b
        - zone: us-central1-c
        - zone: us-central1-f
        platform: GCP
      metadata:
        labels:
          machine.openshift.io/cluster-api-cluster: oadp-92591-q6lgb
          machine.openshift.io/cluster-api-machine-role: master
          machine.openshift.io/cluster-api-machine-type: master
      spec:
        lifecycleHooks: {}
        metadata: {}
        providerSpec:
          value:
            aaa: aaa
            apiVersion: machine.openshift.io/v1beta1
            canIPForward: false
            credentialsSecret:
              name: gcp-cloud-credentials
            deletionProtection: false
            disks:
            - autoDelete: true
              boot: true
              image: projects/rhcos-cloud/global/images/rhcos-416-94-202406251923-0-gcp-x86-64
              labels: {}
              sizeGb: 128
              type: pd-ssd
            kind: GCPMachineProviderSpec
            machineType: n2-standard-4
            metadata:
              creationTimestamp: null
            networkInterfaces:
            - network: oadp-92591-q6lgb-network
              subnetwork: oadp-92591-q6lgb-master-subnet
            projectID: openshift-qe
            region: us-central1
            serviceAccounts:
            - email: oadp-92591-q6lgb-m@openshift-qe.iam.gserviceaccount.com
              scopes:
              - https://www.googleapis.com/auth/cloud-platform
            shieldedInstanceConfig: {}
            tags:
            - oadp-92591-q6lgb-master
            targetPools:
            - oadp-92591-q6lgb-api
            userDataSecret:
              name: master-user-data
            zone: ""