To view Gmail messages in your aggregator, just subscribe to a new feed, enter https://mail.google.com/mail/feed/atom in the URL field, and submit your Gmail address and password. Google Apps users should enter https://mail.google.com/a/your_domain.com/mail/feed/atom in the URL field. You don't need to do a thing from within Gmail; you do it all from within your aggregator.
We recommend setting your aggregator to check for new Gmail messages once every ten minutes.
Keep in mind when you subscribe to your Gmail feed that some aggregators allow you to share, syndicate, or otherwise re-publish the feeds you subscribe to. This means that even though your Gmail feed is private, its contents could be made public through one of these methods. So you'll want to double-check the privacy settings in your aggregator to make sure you aren't sharing any information you don't want to. Specific instructions will vary depending on what aggregator you're using.
Also keep in mind that Gmail messages won't appear in your aggregator unless there are unread messages in your inbox.