IPv6 not supported in k8s service loadBalancerSourceRanges

You are seeing this error because IPv6 cannot be used along IPv4 in k8s (you could run k8s in ipv6-only mode, but this would not work in GCP since GCP does not allow to use ipv6 addresses for internal communication).

GCP VPC docs:

VPC networks only support IPv4 unicast traffic. They do not support broadcast, multicast, or IPv6 traffic within the network; VMs in the VPC network can only send to IPv4 destinations and only receive traffic from IPv4 sources. However, it is possible to create an IPv6 address for a global load balancer.

K8s 1.16+ provides dual stack feature that is in early development (alpha) stage that allows for IPv6 and can be enabled with feature-gates, but since you are using GKE, controlplain is managed by GCP so you can't enable it (and since is alpha feature you probably should not want to).

You can find a bit more about this dual stack feature here:dual-stackand here:validate-dual-stack

Here is the latest pull request I have found on github relating this feature: https://github.com/kubernetes/kubernetes/pull/91824

I think that we can expect that the beta version will appear soon in one of k8s releases but since GKE is about two versions behind the latest relese, I infere that it will take some time before we can use IPv6 with GKE.