apt-get install postfix libsasl2-modules

Add the following lines in /etc/postfix/main.cf:

smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
relayhost = [smtp.gmail.com]:587

Add the file /etc/postfix/sasl_passwd

[smtp.gmail.com]:587 youruser@gmail.com:yourpassword

And run postmap on it

postmap sasl_passwd

restart postfix and your done :)