So quite some time ago Gmail blocked insecure smtp access to your gmail account, even when you turned on tls which previously had been working for a few years.

There is however a workaround for this where you create an email-only login to your gmail account:

1. Open My Account from the Google menu:
   


2. Click the Signing in to Google link:
   


3. Click App passwords:
   


4. You're prompted to verify your login:
   


5. Follow through the options here, under Select app choose Mail (or other Google function -- say Youtube if you're setting up a special login for a media box), select a device from the drop down list or choose Other, then click Generate:
   


6. Copy and paste the created password into your device.  SMTP settings are smtp.gmail.com:465, tls enabled, user is your gmail email address.

HTH!