Your Gadget specification will look something like this:
<?xml version="1.0" encoding="UTF-8"?> <Module> <ModulePrefs height="100" title="My Simple Gadget" description="Test gadget" author="Anonymous" author_email="firstname.lastname@example.org"/> <Content type="url" href="http://yourappid.appspot.com/gadget.html" /> </Module>
In my case I'm using a very simple GWT application. Make sure that it looks good at 300 pixels wide. The height can vary but don't make it too tall because people are less likely to install it if it takes up too much space.
The URL content type approach allows you to make a normal web page using any tools you like and you can debug it like any normal page.
To put it all together simply save your gadget spec into the war directory of your GAE application and deploy it so it's available on a URL like this: http://yourappid.appspot.com/gadgetspec.xml
Now you can test it in the iGoogle Sandbox which (after you've signed up) allows you to test unpublished gadgets. Add this developer gadget to make life easy.
When you're happy with it submit your gadget and you're done.