Hi,
Had an issue setting this up but got it working as there was a restriction on SMTP in cPanel...
THIS WORKS
I have set it up as per the specs here:
https://support.google.com/a/answer/2956491
SMTP Host: smtp-relay.gmail.com
SMTP Port: 587
SMTP Username: no username as google says "IP address provides authentication."
SMTP Password: no password as google says "IP address provides authentication."
SMTP Secure Connection: TLS Legacy
More info on gmail SMTP setup here:
https://support.google.com/a/answer/176600?hl=en