Is OpenPGP vulnerable to the "crypto doom principle"? From reading RFC 2015 (MIME security with PGP), and from looking at the raw format of (GnuPG) encrypted and digitally signed email messages, it seems that first a signature is calculated from the plain text message, and then both are encrypted. 

My question is why isn't this vulnerable to what Moxie Marlinspike dubs the "cryptographic doom principle" (basically, if you have to do any crypto operation before checking the signature, bad things will happen)? I suppose the reason (alongside the devil) will be in the details, in the particular way the "sign-then-encrypt" operation is implemented. But I've been unable to find (or more likely to make sense) of those details. Can anyone shed some light?
The answer (to the title question) is no, due to a slight mixup in terminology. What Moxie says is (emphasis added)


  if you have to perform any cryptographic operation before verifying the MAC on a message you’ve received, it will somehow inevitably lead to doom.


Notice that he says MAC and not signature. The PGP group has discussed this too, so you are not alone in your concern.

In other words, mac-then-encrypt is not the same thing as sign-then-encrypt. Sign-then-encrypt is preferred to encrypt-then-sign. For more discussion on sign-then-encrypt see this question and answers.