Hi, I made a small user-friendly script for a no-questions-asked installation and configuration of postfix with Gmail.

The script configures postfix to relay mail to smtp.gmail.com (which will send your emails to the world).
Check out the https://gist.github.com/3952294
and please tell me if it works for you (or not)

P.S. I checked the script on Ubuntu 12.04, but it should work on previous versions as well.
Also, you should consider purging all postfix configuration with
sudo apt-get purge postfix