Improve speed of GCP SQL Failover
There isn't anything you can do to improve the failover time, but you can add a read replica to have read access to your data while your instance is recovering. 

If you aren't locked down to MySQL, you should also try the Postgres version of Cloud SQL - it uses a faster method for recovery and from my personal experience is generally down for ~30-60 seconds. Each version has it's own page on HA, which talks about how they work: MySQL, Postgres