Setting up Postfix to relay mail to Google's Gmail is an exercise in configuring TLS and SASL. Plus, there is the issue of safely downloading Gmail with a STARTTLS configured Fetchmail; and, if you are using the sslcertck option in your .fetchmailrc (which you should), then, you will need to rehash certificate files.
http://souptonuts.sourceforge.net/postfix_tutorial.html
The above link Gmail on Home Linux Box using Postfix and Fetchmail show you how to do everything - it's a step by step approach with examples, with special notes for Fedora Core users who have Sendmail configured as the default.
Regards,
Mike Chirico