We use Google Cloud DNS for primary DNS zone hosting. As a primary service is seems very functional. However, I cannot see how to allow zone transfers to a list of secondary DNS servers. This would be useful to provide further resilience. For instance, in Europe RIPE will act as a secondary on all delegated reverse DNS zones.
I have searched the web and the Google help for this information but I have not been able to find an answer. are secondary servers supported by Google Cloud DNS?