Bill actually has a tutorials page that shows how you can customize the grid loop. Take a look here – http://www.billerickson.net/contributions/tutorials/
He also has a plugin as well – http://wordpress.org/plugins/genesis-grid-loop/
The above tutorials and plugin should be enough….if not, you can do a Google search on how to customize the grid loop and you’ll find a lot more tutorials that are already written specifically for it.