Ben's Journal: The Anatomy of A Blogger Gadget

Question:

I asked myself, how to write a little blogger gadget.

Answer:

Ben tells us to use the Google API: Ben's Journal: The Anatomy of A Blogger Gadget

The Google Gadget API
Gadgets API Blog
Some Samples

But I really think, you should start with a look at BloggerAPI the
Widget Tags for Layouts and available data container.