The issue with blocking Gmail is that Gmail is a lot of people's only email. This list is a list of domains people use for addresses that definitively aren't their primary email address (since they are disposable/temporary/forwarding addresses).

While Gmail might be a major source of SPAM, they're also the email service most people legitimately use. If you block [email protected], someone might be annoyed, but can sign up with their non-proxied address. If you block Gmail, the person can't just use their "real" address. You're forcing them to create a different email address to use your service - and there's no reason to think that new email address will be from a service that better validates their users.