I realize this is a really elementary question but I can't seem to find a good answer to my question through googling.
I want to set up an email address on my domain, e.g. [email protected]. I think I'd like it to forward to my GMail account.
I'm on Ubuntu and I already have Postfix installed.
I'd suggest using Google apps for this and just setting up forwarding. If you insist on setting up a mail server, check out this guide: http://flurdy.com/docs/postfix/
From the content of your question, I'd suggest that instead of trying to set up a mail server of your own, that you consider setting up a Google apps account for your domain. You can configure either account to forward to the other, as you see fit.
The process is fairly straight forward. As you've already got you domain, you need to sign up to Google Apps, then create a couple of MX records in your registrar's control panel and point them at Google's servers. The whole process shouldn't take longer than a couple of days.
If you want mail for your domain, there needs to be a server handling it and you'll need to setup an MX record on the domain to point to the server's address. If you don't personally setup a server to handle it i'm sure there will be a charge from the company you bought the domain from to add e-mail to your domain. If you get e-mail service from the company you purchased the domain from and pay the monthly fee, there will be a place on your account to set a forward address. Google may do all of this for free but i've never set it up.