Convert encrypted and signed to just signed PGP message

Suppose Bob got a message from Alice encrypted with his public key and signed with her private key. Now he wants to prove to Charlie that he got a message from her with this exact content. The message was created via gpg --sign --encrypt. My idea was that he could decrypt the message and save it with its signature somewhere but I could find no way to achieve this. But since GPG signs the message and then encrypts it afterwards this should at least theoretically be possible. Now how can he do this or do you have any other ideas how Bob can proof the message authenticity to Charlie? Restrictions: - Giving Charlie Bobs private key is (obviously) not an option. - Communication is only possible via email. - Alice cannot be contacted any more so resending the message or Charlie and Alice communicating with each other is not possible. Bob has to work with what he already has.