You can add an existing Kubernetes cluster and then manage it using KKP.
External Clusters page.
Import External Cluster button and Pick Google Kubernetes Engine provider.

You should see the list of all available clusters. Select the one and click the Import Cluster button.
Clusters can be imported only once in a single project. The same cluster can be imported in multiple projects.

Provide Credentials in either of the below mentioned ways:
Select a pre-created preset which stores the provider specific credentials.
Create a preset on your KKP cluster with spec.gke.serviceAccount containing the base64 encoded service account.
Manually enter the credentials ServiceAccount
After user provides all required credentials, credentials will be validated.
Validation performed will only check if the credentials have Read access.
Admin can create a preset on a KKP cluster using KKP Admin Panel.
This Preset can then be used to Create/Import an GKE cluster.
Admin Panel from the menu.
Provider Presets Page and Click on + Create Preset button.

Google Kubernetes Engine from the list of providers.
Create button.

After the cluster is added, the KKP controller retrieves the cluster kubeconfig to display all necessary information. A healthy cluster has Running state. Otherwise, the cluster can be in the Error state. Move the mouse cursor over the
state indicator to get more details. You can also expand Events to get information from the controller.

You can also expand Events to get information from the controller.

You can also click on Machine Deployments to get the details:

When an upgrade for the cluster is available, a little dropdown arrow will be shown beside the Master Version on the cluster’s page. To start the upgrade, just click on the link and choose the desired version:

If the version upgrade is valid, the cluster state will change to Reconciling.
Only one operation can be performed at one point of time. If the replica count is updated then Kubernetes version upgrade will be disabled and vice versa.



Delete operation is not allowed for imported clusters
Delete cluster operation allows to delete the cluster from the Provider. Click on the Delete button.

Navigate to the cluster overview, scroll down to machine deployments and click on the delete icon next to the machine deployment you want to delete.

The KKP platform allows getting kubeconfig file for the GKE cluster.

The end-user must be aware that the kubeconfig expires after some short period of time. To mitigate this disadvantage you can extend the kubeconfig for the provider information and use exported JSON with the service account for the authentication.
Add name: gcp for the users:
users:
- name: gke_kubermatic-dev_europe-central2-a_test
user:
auth-provider:
name: gcp
Provide authentication credentials to your application code by setting the environment variable GOOGLE_APPLICATION_CREDENTIALS.
This variable applies only to your current shell session. If you want the variable to apply to future shell sessions,
set the variable in your shell startup file, for example in the ~/.bashrc or ~/.profile file.
export GOOGLE_APPLICATION_CREDENTIALS="KEY_PATH"
Replace KEY_PATH with the path of the JSON file that contains your service account key.
For example:
export GOOGLE_APPLICATION_CREDENTIALS="/home/user/Downloads/service-account-file.json"