From Gmail Help Center:
How do I view Gmail messages with my aggregator?
You can view Gmail messages in your aggregator by subscribing to a new channel. Enter https://mail.google.com/gmail/feed/atom in the URL field, then submit your Gmail address and password.
Keep in mind that Gmail messages won't appear in your aggregator unless there are unread messages in your inbox.
Update: Bloglines and Google Reader do not appear to support this. It works fine with Onfolio as it prompts for ones userid and password.