Probably just use Apple Mail.
1. Use POP3 to download all Comcast mail to Apple Mail on your Mac.
2. Use IMAP to upload it all to GMAIL.
That's method 1.
Method 2.
1. Configure GMAIL to use POP3 to pull in all your Comcast mail.
2. There is no step 2.
Post tonight if you need to hear more about either of these methods, I have a little time watching the games, plus a comcast and a gmail account...