The Problem - One Login Only
Magento uses a single login to connect to the Google Merchant Center and upload product data. However, many eCommerce managers and webmasters will have a multi-client account at Google Merchant Center and there's no obvious way in Magento to associate the data feed with one domain or store, and not another.
The Solution - Specify the Client ID
Use the login feature for Google Base in Magento, but modify a few of Magento's files to specify which store should receive the product feed.
First modify the fiile lib/Zend/Gdata.php script around line 200, so that the client ID is inserted into the URL:
That should be enough to connect to the Google Merchant Center and properly submit a feed. However, we'll need to make an additional modification in order to ensure that the "Google Base --> Manage Attributes" page is still accessible in Magento.
Navigate to and modify app/code/core/Mage/GoogleBase/Model/Service/Feed.php (line 37):
This isn't a perfect solution, but it's a quick and easy way to connect Magento to multi-client accounts at Google Base/Merchant Center. Hope it's helpful for you!
This article is based on the forum post How Do I Setup Google Base?. Thanks are due in particular to forum user Jason J for the fix.