I did some consulting work on a site quite a while ago that used xcart, at the time it seemed to work fine. Checking their site I found this:
Qualiteam Software news: Certified integration of Google Checkout with X-Cart
But I haven't looked into the specifics of implementing it.
The tax gift cards seem to work properly...hate to ask, but from the page you linked there's no way to add anything to the cart. Am I missing something?
Kind of curious, was thinking about using xcart in a new project starting next year....