In their docs the links all seem to resolve to https://developers.google.com/apps-marketplace/ - where there doesn't seem to be any call to action. How does one test a Gmail Contextual Gadget? Where does one upload the manifest and gadget spec?
It is now in the Google Developers Console and you need to enable the Google Apps Marketplace SDK API. Under the Configuration tab, near the bottom, there is an Extensions header with a checkbox for Gmail contextual gadget extension. Clicking this opens the options to enable a Gadget URL and start developing.
As far as I can tell, there is no documentation for this anywhere on the interwebs, so I hope this is helpful for someone who struggles with this like I did :-)