the code:

def create_cluster(self):
        conf = self.config.get_object('gke')['cluster']
        # Create a GCP resource (GKE Cluster)
        return container.Cluster(
            conf['resource-name'],
            name=conf['name'],
            initial_node_count=conf['node_count'],
            network=self.network,
            subnetwork=self.subnet.id,
            # master_auth=container.ClusterMasterAuthArgs(
                
            #     username=conf['username'],
            #     password=conf['password']
            # ),
            logging_service="<http://logging.googleapis.com/kubernetes|logging.googleapis.com/kubernetes>",
            monitoring_service="<http://monitoring.googleapis.com/kubernetes|monitoring.googleapis.com/kubernetes>",
            node_config=container.ClusterNodeConfigArgs(
                oauth_scopes=conf['oauth_scopes'],
                machine_type=conf['node_machine_type'],
            ),
            node_version=conf['node_version'],
            remove_default_node_pool=True,
            location=conf['location'],
            private_cluster_config=container.ClusterPrivateClusterConfigArgs(
                enable_private_endpoint=True,
                enable_private_nodes=True
            ),
            enable_autopilot=False,
            cluster_telemetry=container.ClusterClusterTelemetryArgs(
                type='ENABLED'
            )
        )

I was using the existing network & subnet. Following code return the existing subnet:

from pulumi_gcp import compute

class Subnet:

    def __init__(self, subnet_name, subnet_id) -> None:
        self.subnet_name = subnet_name
        self.subnet_id = subnet_id

    # Get existing sub network
    def get_subnet_instance(self):
        return compute.Subnetwork.get(self.subnet_name, self.subnet_id)

Hi, this has been resolved.