From my understanding, PGP supports the generation of a signature from plaintext using the sender's private key. Both the plaintext and the signature are encrypted using the session/symmetric key, and the session/symmetric key is encrypted using the receiver's public key.
I assume the reason the signature is created from the plaintext (aside from ease of logistics) is so this signature can't be removed and replaced with another signature. Is that correct, or is there another reason? By signing before, the receiver runs the security risk of decrypting an unauthenticated message.