I am very surprised that there isn't a google checkout addon available in the marketplace. This unfortunately means you'll need to code one up yourself. I'd suggest taking the built-in PayPal "Simple Integration Method" and using that as a starting point -- google checkout and paypal SIM work in a similar way so hopefully it won't require too many changes.
Here's some basic documentation on creating your own payment type:
http://www.concrete5.org/marketplace/addons/ecommerce/documentation...

Anyone else care to share? I offer payment

I'm just curious if anyone got this to work with Google Checkout and, if so, what kind of hassles you might have come across in doing so. Thanks.