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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>