Yes, this is enabled for all VMs running on Compute Engine (which includes GKE VMs), however the in-guest iptables &c. bits add non-trivial overhead (I don't have numbers handy, apologies).
Regarding in-guest iptables -- there's not much we can do on GKE/Kubernetes about that. I serve about 1.2M requests per second on my GKE cluster through GLB and see this overhead very well.
FWIW I also look forward to IPVS in k8s 1.9 that should improve this slightly.