This test was completed in December and the tests proved that multicast does not work with GKE networks. The test was performed by deploying a 3 node GKE cluster with a debian image and overlay network using weave as documented here: https://www.weave.works/blog/weave-net-kubernetes-integration/ and https://cloud.google.com/kubernetes-engine/docs/how-to/network-policy
Thereafter we tried to move traffic over the multicast IP range on the weave network using this: https://taosecurity.blogspot.com/2006/09/generating-multicast-traffic.html
We found that if our pod ended up running on the same node that we could send packets over the multicast overlay but in the case of the pods running on a different node, multicast could not receive data packets transmitted over the same overlay network.
Conclusion: GKE does not support multicast, and we will have a follow-up discussion for potential replacement technologies.