GKE sets up the kubernetes nodes for you. The load balancer should work straight out. Compare your yaml with the one below. And check for its status running kubectl get svc. Are you getting a ClusterIP IP? What happens if you change the type to NodePort, is you app reachable through the nodes IPS?

apiVersion: v1
kind: Service
metadata:
  name: example-service
  namespace: default
  labels:
    app: example-service
spec:
  type: LoadBalancer
  ports:
  - name: "port"
    port: 8000
    targetPort: 8000
  selector:
    app: example-service