Google Checkout only supports pixel-based tracking solutions. This means that tracking merchants (like Spring Metrics) must provide website owners with a 1x1 pixel image to place on their site's pages. The pixel then captures information about your visitors and relays it to the tracking system. For more information about this, please see this Google article.
However, we can help you track how many people click on your Google Checkout button and leave your site to checkout. Of course, there will always be people that click the button but do not complete the purchase and therefore skew the conversion data. But if you want to track clicks on the Google Checkout button, see this article for information on how to do that: http://j.mp/rpGYqI