Google Cloud Load Balancing is implemented as a single Anycast IP, which provides cross-region load balancing and automatic multi-region failover.
With Anycast IP network, a single IP is allocated for serving the incoming request and the request is routed to the nearest Google server region/location. Once the traffic is inside the Google Cloud network, it is routed through the Google private network infrastructure to further serve the request quickly and minimize latency.
To talk about the implementation briefly, Anycast is realized using the standard BGP (Bridge Control Protocol) protocol, which is used for advertising routes and all internet routers are eventually aware on how to reach the shortest path/location for the Anycast IP. For instance, if the incoming request is originating from India and if a public route is available for US as well as India region, India region would be picked up for serving the request. This is handled transparently by the internet network.
The other way for implementing a Load balancer is through DNS-based approach. AWS uses DNS-based approach. For more details refer to – How does AWS implements Cross Region Load Balancing