Short answer “No”, it is not possible to set the 'gtid_mode' to off on a Cloud SQL 2nd Generation instance. In fact, modifying this parameter requires the “Super” privileges which are disabled for Cloud SQL instances as stated on this MySQL root user account FAQ.
Note that 'gtid_mode' is used when binary logging is enabled to prevent data loss during replica creation and failover. It is recommended you migrate your queries to work with GTID as this is now a default setting on all Cloud SQL instances.
Lastly, if you require super level access, you can host a custom SQL instance on Google Compute Engine.