Any developer who's ever attempted to integrate Google Checkout into their website knows that it is a rather complicated task. The sample code (at least for PHP), while very well written, lacks full documentation on how to use it, and even then it is still more complicated than it needs to be, with the simple task of sending the cart requiring somewhere around a dozen classes. The purpose of GCOi is to provide a simple interface class for the Google sample code in order to make intergration simpler and easier for inexperienced developers to use.
If you have any questions or comments, you can contact the project admin, Brad Koch, through his developer profile at https://sourceforge.net/users/bjk2007/.