I'm not sure when/if they'll have their own shopping cart. Currently they provide the resources for programmers to integrate Checkout into virtually any shopping system, but I don't believe that they currently have their very own cart (could be mistaken).
GNU is an abbreviation for General Public License. Which means that the software is free to use and in most cases free to modify and redistribute.
Our free hosting services are a bit of on the technical side and require some ftp and html experience, so it may not be right for you. Personally I haven't had a chance to review
PSU's VirtueMart with Google Checkout, but I presume that it's an easy to use system since Joomla itself has a simple and easy-to-use interface.
PSU's VirtueMart link is available on the left side menu.
Kind Regards,
AlphaLeap