You purchase your own domain name and use that domain name as your email address. For instance, if I had an email address that was me@afandian.com; the afandian.com would be a custom domain. It's not routed to @gmail.com, it's routed to @afandian.com. Now in practice you can have a custom domain and still have it managed by Google's Mail servers; but it's the domain name itself that sends up the flags.