Am I right, that Google Mail’s default canonical domain name is:
and that’s the only one? Are there any country-specific TLDs used?
I am not sure of your motivation for asking this, but you may be interested in Google Apps. With it, you can use the Gmail software (spam filtering, web interface, etc) on your own domain. The only cost associated with this is in registering your domain name. (There are also paid versions of the Google Apps service with more features, but I find the free version sufficient for my family and myself.)