Well, first you need to go into your gmail account preferences (using a supported browser, like firefox), and enable POP3 access. For my account (which I assume is the same as other gmail accounts), it's under the "Forwarding and POP" section - enable "Enable POP for all mail". Then look at the link " Configuration instructions", and select "Other".
I'd recommend using pine rather than mutt - it's easier to use. It was actually the first email client I ever used, back in the early 90s.
The version I found in the ubuntu repositories is called alpine, which looks like the pine I used to use. I don't know if it is exactly the same as the version you have.
Alpine 0.99 built Sun May 6 09:55:42 GMT 2007 on palmer
Google's POP3 access requires ssl, and as far as I can tell, Alpine does not support this, so you need to use an external POP3 client, such as fetchmail. Please read the user documentation for fetchmail and [al]pine to work out how to configure it.
As you can tell, this is quite a bit of effort - why not simply use firefox?