is yaml in this case not supported nad will I have to use python?

You should be able to use YAML if desired: https://www.pulumi.com/registry/packages/kubernetes/api-docs/core/v1/namespace/

In the sample code, it seems like you are only creating the gcp gke resource. You then need to create a separate K8s namespace resource. Here is a yaml example that is solely deploying k8s resources - using a kubeconfig from a stack reference - but hopefully this helps. https://github.com/pulumi-demos/examples/blob/main/yaml/k8s-guestbook-app-yaml/Pulumi.yaml