A b2a file is basically an Adobe Flash SWF file that needs to contain a pre-determined set of functions defined strictly in ActionScript 3. [b²] Gmail Notifier uses these functions to communicate with the animation. Anyone who know how to use Adobe Flash - on a beginners level - can easily create notifier animations for [b²] Gmail Notifier.
See the attached PDF for details, or the attached ZIP archive for a few working examples! You will need ADOBE FLASH CS4 to open examples!