Unless I am wrong, gpg protects the content of the mail, the so called body not the header sometimes called the envelope. Accordingly all header fields can be spoofed. Just imagine I spoof the Reply To: field of a mail. Depending on the case, you would respond to me and not to the sender of the email. Gpg will not protect you against this. It will just prevent me to read the message.

My best knowledge of this matter is: IPV6 is designed to protect email traffic. It supports encryption and certificates at protocol layer level. Unfortunately the only implementation of IPV6 I have seen until now are at Internet-backbone level. Most of the mail server still use IPV4 and do not implement encryption and certificates for a simple reason: you partially close your inbox if you do and so you might miss some emails of potential clients.