All is by design. Google Checkout is an alternative CHECKOUT system, not just a payment module. Customer does not have to login to Zen Cart to use checkout via Google Checkout. So GC is available ONLY via login and shopping cart pages, (and GC option is not available on regular zen cart payment method page, which you would have to be logged into Zc to access). Hope this makes sense. Yes it is different from normal checkout and can be confusing to customers and shop-owners. I wish there was a better way. This point you make probably should be highlighted in the readme.
Woody
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