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To access it, you’ll first need to enable custom gadgets in Gmail, which can be done from the Labs page under Settings. From there, you just insert the URL via the news "Gadgets" link you'll have under Settings - http://www.twittergadget.com/gadget_gmail.xml - at which point you’ll be prompted for your Twitter login.
Fittingly, users are debating the usefulness of this on Twitter, with some arguing it’s far too small to be functional, while others think it makes Gmail even more of a sticky one-place-for-everything application.