I was very excited when I saw that you could now add third-party Gmail gadgets through a new labs feature. My obsession is opening up the closed silos that hold our email, so I hoped this would give external developers access to messages. Unfortunately for me, the gadgets interface gives you access to the UI of Gmail, but there's no new API for looking at emails. That is very handy for tools like Remember The Milk, which make sense as part of your mail interface, but don't need to know anything about your messages, but I'm still stuck with IMAP and 'Lockdown in sector 4 (Failure)'.
If you want to build your own Gmail gadgets, it's essentially the same process as building one for iGoogle. Have a look at the docs for gadgets.*, and be aware that this is still a labs feature, so your users will have to jump through several hoops to install the add-in. There is one ray of hope; since you can now inject your own html into the Gmail interface, it should be possible to screen-scrape the email information that's currently displayed. Xoopit is doing exactly this in their Firefox plugin, so they could probably create an equivalent Gmail gadget and have a lower-friction adoption path.