As you mentioned, GCE persistent Disk snapshots will backup all the data that your Persistent Disk holds at the time they are taken.
If you have used Cloud Launcher to deploy your solution,
the database and all the other services lives in the same VM. You can verify that by connecting in the VM and checking the configuration file.
i.e
$ cat /var/www/html/app/etc/env.php | grep localhost -A5 -B4
'connection' =>
array (
'default' =>
array (
'host' => 'localhost',
'dbname' => 'magento',
'username' => 'magento',
'password' => xxxxx,
'model' => 'mysql4',
'engine' => 'innodb',
displays the configuration of the connection to the database server (in this case the same host). If you deployed Cloud Launcher Bitnami's Magento version, configuration files will have another location but still be stored in the persistent disk.
Therefore taking a snapshot will be also backing up your database and system configuration.
If you have altered the configuration files and your database server is different than the local VM, you need to backup the respective server. For example, if you are using a Cloud SQL server to host your database you can refer to this article.
As per every backup policy, I would suggest taking the snapshot and spinning a new VM from it to test it.