I have a NAS 316 running  the latest software and the email alerts to my gmail account stopped.  I found out that Gmail dropped the option of allowing less secure apps such as the NAS.  If you have 2 factor authentication enabled then you can use the apps password that is referenced in this article 

ReadyNAS OS6 Email alert configuration for GMail

If you do have 2 factor authentication turned on then you can use the sign in with google option under the settings menu. To get the sign in with google option to show up, clear the email settings ( under the settings tab) enter your email address and use the sign in with google option.