i assume you set up your service by setting the service's type to LoadBalancer? It's an unfortunate limitation of the way incoming network-load-balanced packets are routed through Kubernetes right now that the client IP gets lost.
Instead of using the service's LoadBalancer type, you could set up an Ingress object to integrate your service with a Google Cloud HTTP(s) Load Balancer, which will add the X-Forwarded-For header to incoming requests.