I have added my gmail account to the comcast email accounts, so that I can check my gmail while on the comcast email page.
The problem is, that I have to reauthorize the account daily. The setting does not seem to be saved day to day. It works upon initial authorization, but reauthorization is required about 85% of the time. Once in awhile the settings hold for a day.
Try deleting the account from the Comcast interface, then re-create it again from scratch. I have my Gmail account set up in the mail fetcher and it works just fine with no issues.