Extend proxy-network troubleshooting for GKE (kedacore#152)

content/troubleshooting/proxy-network.md

@@ -29,7 +29,7 @@ kubectl get apiservice v1beta1.external.metrics.k8s.io -o yaml
 
 If the status is `False`, then there seems to be an issue and proxy network might be the primary reason for it.
 
-### Follow these steps:
+### Solution for self-managed Kubernetes cluster:
 
 Find the cluster IP for the `keda-metrics-apiserver` and `keda-operator-metrics`:
 
@@ -52,3 +52,9 @@ sudo systemctl restart kubelet
 ```
 
 Check the API service status and the pods now. Should work!
+
+### Solution for managed Kubernetes services:
+
+In managed Kubernetes services you might solve the issue by updating firewall rules in your cluster. 
+
+E.g. in GKE private cluster [add](https://cloud.google.com/kubernetes-engine/docs/how-to/private-clusters#add_firewall_rules) port 6443 (kube-apiserver) to allowed ports in master node firewall rules.