The full text of Gaudium et Spes is available online, but I will be focusing on 27 that is the source of Shea's arguments:
Furthermore, whatever is opposed to life itself, such as any type of murder, genocide, abortion, euthanasia or wilful self-destruction, whatever violates the integrity of the human person, such as mutilation, torments inflicted on body or mind, attempts to coerce the will itself; whatever insults human dignity, such as subhuman living conditions, arbitrary imprisonment, deportation, slavery, prostitution, the selling of women and children; as well as disgraceful working conditions, where men are treated as mere tools for profit, rather than as free and responsible persons; all these things and others of their like are infamies indeed. They poison human society, but they do more harm to those who practice them than those who suffer from the injury. Moreover, they are supreme dishonor to the Creator.
Now I agree with all of that, but when read in any kind of textual or historical context it clearly doesn't mean what Mark says it does, namely that torture is in of itself an intrinsic evil. In fact, the idea that Pope John Paul the Great is rounding off a list of intrinsic evils is in of itself at odds with Church teaching given that a number of the items mentioned (notably deportation, slavery, and poor working conditions) are not themselves regarded as intrinsic evils by the Magisterium. Particularly, if any attempt to coerce the will is to be considered an intrinsic evil it would seem that the Magisterium is essentially adopting the Jain view of pacifism, which is to say one quite at odds with what the Church actually believes here in the real world. I'm not even going to get into the fact that what is considered subhuman working conditions to us in 2006 would not have been 100 years ago, let alone 500 or 1,000. (italics by VJM)
Ultimately, the view that you are left with is a view from John Paul the Great that does not come off terribly favorable towards torture but which stops well short of labeling it as an intrinsic evil. Indeed, in terms of development of doctrine the Gaudium et Spes quote would seem to be quite consistent with taking up a view of torture that is either analogous to that held by the Church on the death penalty or should only be used in very exceptional "ticking time bomb" cases. And if Mark still believes that these only occur on 24, I would ask him to Google Abdul Hakim Murad or Rashid Rauf for two very specific examples. The comparison that he will no doubt be apt to make to that of the "safe, legal, and rare" argument frequently invoked by abortionists is in this context a category mistake, since abortion is explicitly defined as an intrinsic evil and has always been recognized as such.
Now, as I've noted in the past, none of this affects Shea's policy argument to the extent that he has one. There is a very good argument to be made (as Father Neuhaus has) that something that only becomes morally licit in exceptions should never be institutionalized. There are also very good arguments to be made on banning torture in any and all circumstances for purely secular reasons. Mark is more than entitled to make those arguments, but if he bothered to recognize what Gaudium et Spes actually says he would have to do so in a far less self-righteous fashion.
UPDATE FROM VJM: I don't think, Torquemada, that one can overemphasize the sentence I have emphasized in italics. The very term "subhuman" when used in reference to a material standard, is about as historical and contingent as a term gets. How can that possibly square with the language of "intrinsic evil," which means that circumstances and cases never matter? Even a child knows that material circumstances and "needs" are relative across time and place. To put it kindly, calling "subhuman living conditions" an "intrinsic evil" is not a statement that can be taken seriously.
And everything I said about "subhuman" housing, rinse and repeat for "disgraceful working conditions."
This suggests to me one of two things (I'm agnostic on which, for the moment): either (1) the things mentioned are intrinsic evils, but some (including potentially, and I wopuld argue actually, "torture") have no ahistoric meaning and thus have to be understood and defined according to circumstance; (2) the whole text is a lofty flight of rhetoric, meant to be taken prophetically (and certainly infallible as such), not as stating anything philosophically precise.