# visit https://accounts.google.com/DisplayUnlockCaptcha and click to allow access

# edit /etc/monit/monitrc to include the following

  set mailserver smtp.gmail.com port 587
       username "whoever@gmail.com" password "whatever"
       using tlsv1
       with timeout 30 seconds
       
# run the following to validate access

  /etc/init.d/monit restart
  monit validate