istio: Readiness probe failed: HTTP probe failed with statuscode: 503

How to fix this - if possible?

kubectl describe pods istio-ingressgateway-66c8bbb77b-j99j6 -n istio-system
...
Events:
  Type     Reason     Age                       From                                                Message
  ----     ------     ----                      ----                                                -------
  Warning  Unhealthy  2m11s (x94307 over 2d4h)  kubelet, gke-mini-default-pool-15a06374-dftr  Readiness probe failed: HTTP probe failed with statuscode: 503

kubectl get pods -n istio-system                                                   
NAME                                    READY   STATUS    RESTARTS   AGE
istio-ingressgateway-57896bd575-cv2p9   0/1     Running   0          4h3m
istio-pilot-74c44c647f-hrz65            1/1     Running   0          4h3m

Environment: Google Kubernetes Engine(GKE)

kubectl version                                                                                                                                   130 ↵
Client Version: version.Info{Major:"1", Minor:"15", GitVersion:"v1.15.9", GitCommit:"2e808b7cb054ee242b68e62455323aa783991f03", GitTreeState:"clean", BuildDate:"2020-01-18T23:33:14Z", GoVersion:"go1.12.12", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"15+", GitVersion:"v1.15.9-gke.24", GitCommit:"39e41a8d6b7221b901a95d3af358dea6994b4a40", GitTreeState:"clean", BuildDate:"2020-02-29T01:24:35Z", GoVersion:"go1.12.12b4", Compiler:"gc", Platform:"linux/amd64"}

istioctl version
client version: 1.5.1
control plane version: 1.5.1
data plane version: none

Installed from https://knative.dev/docs/install/installing-istio/#installing-istio-without-sidecar-injection:

# A lighter template, with just pilot/gateway.
# Based on install/kubernetes/helm/istio/values-istio-minimal.yaml
helm template --namespace=istio-system \
  --set prometheus.enabled=false \
  --set mixer.enabled=false \
  --set mixer.policy.enabled=false \
  --set mixer.telemetry.enabled=false \
  `# Pilot doesn't need a sidecar.` \
  --set pilot.sidecar=false \
  --set pilot.resources.requests.memory=128Mi \
  `# Disable galley (and things requiring galley).` \
  --set galley.enabled=false \
  --set global.useMCP=false \
  `# Disable security / policy.` \
  --set security.enabled=false \
  --set global.disablePolicyChecks=true \
  `# Disable sidecar injection.` \
  --set sidecarInjectorWebhook.enabled=false \
  --set global.proxy.autoInject=disabled \
  --set global.omitSidecarInjectorConfigMap=true \
  --set gateways.istio-ingressgateway.autoscaleMin=1 \
  --set gateways.istio-ingressgateway.autoscaleMax=2 \
  `# Set pilot trace sampling to 100%` \
  --set pilot.traceSampling=100 \
  --set global.mtls.auto=false \
  install/kubernetes/helm/istio \
  > ./istio-lean.yaml

kubectl apply -f istio-lean.yaml

Logs

kubectl -n istio-system  logs istio-ingressgateway-57896bd575-cv2p9
2020-04-04T07:47:52.944158Z	info	Envoy proxy is NOT ready: config not received from Pilot (is Pilot running?): cds updates: 0 successful, 0 rejected; lds updates: 0 successful, 0 rejected
2020-04-04T07:47:54.943944Z	info	Envoy proxy is NOT ready: config not received from Pilot (is Pilot running?): cds updates: 0 successful, 0 rejected; lds updates: 0 successful, 0 rejected
2020-04-04T07:47:56.944064Z	info	Envoy proxy is NOT ready: config not received from Pilot (is Pilot running?): cds updates: 0 successful, 0 rejected; lds updates: 0 successful, 0 rejected
2020-04-04T07:47:58.945674Z	info	Envoy proxy is NOT ready: config not received from Pilot (is Pilot running?): cds updates: 0 successful, 0 rejected; lds updates: 0 successful, 0 rejected
2020-04-04T07:48:00.944137Z	info	Envoy proxy is NOT ready: config not received from Pilot (is Pilot running?): cds updates: 0 successful, 0 rejected; lds updates: 0 successful, 0 rejected
2020-04-04T07:48:02.944018Z	info	Envoy proxy is NOT ready: config not received from Pilot (is Pilot running?): cds updates: 0 successful, 0 rejected; lds updates: 0 successful, 0 rejected
2020-04-04T07:48:04.943958Z	info	Envoy proxy is NOT ready: config not received from Pilot (is Pilot running?): cds updates: 0 successful, 0 rejected; lds updates: 0 successful, 0 rejected
[Envoy (Epoch 0)] [2020-04-04 07:48:05.780][19][warning][config] [bazel-out/k8-opt/bin/external/envoy/source/common/config/_virtual_includes/grpc_stream_lib/common/config/grpc_stream.h:91] gRPC config stream closed: 14, no healthy upstream
[Envoy (Epoch 0)] [2020-04-04 07:48:05.780][19][warning][config] [bazel-out/k8-opt/bin/external/envoy/source/common/config/_virtual_includes/grpc_stream_lib/common/config/grpc_stream.h:54] Unable to establish new stream