The 8.8.8.8 DNS servers ARE NOT authoritative DNS servers, they are recursive resolvers. When your registrar is asking you for a DNS server for your domain, that has be an authoritative server that is actually storing and managing the DNS data for your zone. You SHOULD NOT use 8.8.8.8 or other Google public DNS IP addresses (under any name) as your domain's nameservers, this won't work, Google public DNS does not have the data to answer queries about your domain unless they're hosted at some actually authoritative DNS server.