Configuration settings to enable FreeNAS to send alerts to Gmail.
Under From e-mail, replace <hostname> with the hostname found under System Information. Under Username, replace <first>.<last>@gmail.com with the Google mail address. If you use two-factor authentication, remember to use an application password for Gmail.
There are other places in FreeNAS where mail notification settings have to be configured. For more information, refer to the post FreeNAS Mail Notification Settings.