With the latest mod version 1.3RC2 (released 04/16/2007) Zen Cart should receive order/transaction data from Google Checkout servers, write the data to your ZC database, and thus order info should be viewable within your ZC admin UI (orders pages), just like orders processed through normal ZC checkout.
Do you have your responsehandler URL and call back method configured correctly?
Have you attempted to set responsehandler URL to http when testing in the sandbox?
Does anything show up in your local Google Checkout error log or any errors in your Sellers account (Settings > Integration)?
Woody
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