How to sync easily between a master database from Google Cloud SQL and a master database from Amazon RDS
Theoretically, just adding replication masters to the two servers should work over the MySQL protocol, and without needing any additional configuration.

You can review the procedures at the below links for details on each side of the process:


AWS - How can I replicate an Amazon RDS for MySQL instance by using binary logs to an on-premises environment?
GCP - Configuring external replicas
GCP - Replicating from an external server