You can add an existing Google KubeOne cluster and then manage it using KKP.
Navigate to KubeOne Clusters page.
Click Import KubeOne Cluster button.

Google provider.

Select a pre-created preset which stores the provider specific credentials.
Manually enter the credentials Base64 encoded Service Account used to create the KubeOne cluster you are importing.
GCP Service Account should be Base64 encoded.

Import KubeOne Cluster.