Chapter 13 - Continous Deployment with Spinnaker requires some revision
As helm v3 was released last year and GKE environment deploys the cluster without Tiller service, the described way of deploying Spinnaker in the documentation is not valid anymore. Github chart also has been deprecated and fails with some "Go" error due to this reason. Google also offers the opportunity to deploy Spinnaker with its own webpage through Cloud Shell (https://cloud.google.com/docs/ci-cd/spinnaker/spinnaker-for-gcp#install_spinnaker_on_google_cloud_platform) but it creates a new K8s cluster for it.
It looks like the new way of getting the chart is with the helm hub (https://artifacthub.io/packages/helm/spinnaker/spinnaker) and it has also the instructions to be installed