Below is information on accessing GCP instances and GKE remotely.
To connect to a GKE cluster with kubectl run gcloud container clusters get-credentials <cluster-name> --region us-central1 --project <project-id>. Replace cluster-name and project-id.
If you are not authenticated to gcp run gcloud init to set your GCP project and login. If you are logged in and just need to change project rungcloud config set projec <project-id>