Sign into your Gmail account. Click on the gear icon in the upper right hand corner of the screen. Then click on Settings>>>>Accounts and Import>>>>>Import Mail and Contacts. Then just follow the prompts.
OK I didn't know you had local folders to move. That is a different scenario. The best thing to do is:
a) Set up your Comcast account to use the IMAP protocol. They are running it in beta currently.
b) Start using an email client like Windows Live Mail or Mozilla Thunderbird, or Outlook if you have it already.
You can get the sign-up for the IMAP beta here-------------
After you register the account, you are supposed to get a confirmation about 24 hours later, and the whole process takes up to 72 hours to complete. Once it's up and running, you will then need to set up both the Comcast and Gmail accounts in the client of your choice.
The new settings will be:
Thunderbird will configure the accounts automatically for you. I'm not sure about Outlook or WLM.
Then you will have your emails on your computer no matter who your ISP is, or other email provider you may have. There really is no easy way to copy local folders from one webmail program to another, and both Comcast and Gmail are webmail. This is one of the many reasons a lot of us in here recommend using a client program for any email account that's important.
When you set up IMAP compliant accounts in a client, all your folders should appear both in the webmail and in the client. Gmail is also IMAP compliant. Comcast is POP3 capable by default, which is why you would need to sign up for the beta.
<after settting up IMAP>
Then you will have your emails on your computer no matter who your ISP is, or other email provider you may have.
IMAP is similar to webmail; you see the emails that are stored on the server, but just seeing them doesn't download them to your computer. You can download emails to your computer via IMAP, and you can put such downloaded emails into a folder such that another IMAP service will "synch" them, but it doesn't "just happen". You have to explicitly copy/paste emails from one folder to another.