connection error adding gmail

Trying to add gmail to apple mail to my Mac Mini with OS X 10.11. I had it there once but the smtp went off line - I tried everything I could with no luck so I figured I would delete it and start over. Now when I click to add Gmail I get this prompt:

Connection Insecure

Failed to verify the server certificate. This could be because of your network configuration or your proxy settings.

Please advise