If you try to follow the instructions in Configuring Email Notification with GMail SMTP, you get a security alert from Google:
Google just blocked someone from signing into your Google Account firstname.lastname@example.org from an app that may put your account at risk. Less secure app
Don’t recognize this activity?
If you didn’t recently receive an error while trying to access a Google service, like Gmail, from a non-Google application, someone may have your password.
SECURE YOUR ACCOUNT
Are you the one who tried signing in?
Google will continue to block sign-in attempts from the app you’re using because it has known security problems or is out of date. You can continue to use this app by allowing access to less secure apps, but this may leave your account vulnerable.
So your choices are:
Don’t have email notifications.
Dumb down the security of your gmail account.
Find some other email server you can send to.
Peplink needs to update the firmware to support more robust email authentication.