Google Cloud SQL Replication Delay
I have seen this happens a lot in Amazon RDS. This occurs with row-based replication when a Master get hammered with lots of writes and they get serialized to one I/O thread.

What you should do is dynamically change the following:

SET GLOBAL innodb_flush_log_at_trx_commit = 2;


Problem is you need the SUPER privilege, which neither Amazon RDS nor Google CloudSQL allow. For Amazon RDS, you change the value in the DB Parameter Group (the list of server options for the MySQL Instance). If you can change the dynamic option through some option group in Google CloudSQL platform, this is the option to change. When replication catches back up, change back to 1.

The answer posted by Rick James would be great if you had multiple databases in the CloudSQL instance. He said


  All operations going into a single database? Any chance of upgrading past 5.6; multi-threaded replication may help.


Try change the dynamic option for now to get rid of the current replication lag, and try his suggestion of going with MySQL 5.7 and setup multithreaded replication as the long-term solution.