I've been struggling with this too some time ago. Since the company where I work uses Google for enterprises I was able to contact the 'business' part of Google. They informed me that from a dynamic IP they only accept relaying if authenticated. We use fixed IP's so I was able real soon to get it up and running. However in my search for answers I stumbled upon this site Postfix Gmail SMTP relay
. With those instructions I was able to set up my 'home mail server' and it connects and relays all mails to my personal gmail account.
Hope this helps you out.