Easy.
Purge postfix and reinstall it, create a sasl_passwd in /etc/postfix, and hash it then add this to the end of main.cf:
Code:
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
# Secure channel TLS with exact nexthop name match.
smtp_tls_security_level = secure
smtp_tls_mandatory_protocols = TLSv1
smtp_tls_mandatory_ciphers = high
smtp_tls_secure_cert_match = nexthop
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
relayhost = smtp.gmail.com:587
Restart postfix and you should be good to go.
I used the same steps to get it up on CentOS, but had to modify the code to point to where Ubuntu stores its SSL certs.
See here for more-or-less step-by-step directions, just replace yum with apt-get.