I wrote this for myself. Since I have no memory, I write everything down. If it helps you -- great. if you run into problems GTS. I have no answers.
I have postfix mapped to a mysql database with three tables: domain, user, alias.
the complete postfix tutorial is here blogging dragon
What he doesn't address, the complexities of forwarding to Gmail, I will below. Gmail is merciless. If your server isn't getting a 8 or greater at mail-tester.com your email will never see an inbox.
we are assuming you have crossed that hurdle.