If your domain uses a CNAME record with a destination of ghs.googlehosted.com, then it's automatically accessible using both IPv4 and IPv6 (with the exception of users of DNS resolvers to which Google does not return AAAA records).
If your application requires a specific protocol version, Google also provides IPv4-only, IPv6-only and dual-stack targets unaffected by the list of DNS resolvers above. The available CNAME targets are:
- ghs.googlehosted.com: Automatic (default)
- ghs4.googlehosted.com: IPv4 only
- ghs6.googlehosted.com: IPv6 only
- ghs46.googlehosted.com: IPv4 and IPv6