I think the general difference is, in AWS you provision your own "private" little loadbalancer instance, and they have logic on how little or big (in terms of compute, bandwith etc) this specific loadbalancer needs to be, and resize it constantly for you.

Google runs a single gigantic distributed loadbalancer, and simply adds some rules for your specific traffic to it. All of the compute and bandwith behind this loadbalancer is able to help serving your traffic spike.

Google's load balancer is a SDN (software defined networking) setup that basically runs as software on their normal computing clusters that power the rest of their services. They have plenty of capacity already handling all the other traffic so there's no real difference in handling a few more requests, unlike AWS which manages custom instances and resizing just for your ELB.

GCP LB design details: https://cloudplatform.googleblog.com/2016/03/Google-shares-s...