Annotate the nginx controller service so the LB provisioner, gives you an internal IP Address.

Try this on your nginx-ingress chart values.yaml:

controller:
  service:
    annotations:
      cloud.google.com/load-balancer-type: "Internal"

Reference:

• https://github.com/helm/charts/tree/master/stable/nginx-ingress#configuration
• https://cloud.google.com/kubernetes-engine/docs/how-to/internal-load-balancing