In this sense, what constitutes the fixity of the body, its contours, its movements, will be fully material, but materiality will be rethought as the effect of power, as power's most productive effect. And there will be no way to understand "gender" as a cultural construct which is imposed upon the surface of matter, understood as "the body" or its given sex. Rather, once "sex" itself is understood in its normativity, the materiality of the body will not be thinkable apart form the materialization of that regulatory norm. "Sex" is, thus, not simply what one has, or a static description of what one is: it will be one of the norms by which the "one" becomes viable at all, that which qualifies a body for life within the domain of cutural intelligibility.
What the body is, is constructed within dense complex social networks, regulatory norms, etc. Meaning is constructed in those networks and intelligibility and unintelligibility as well. Social construction is not 'false' or 'inessential' or even volunteeristic, but at the same time, there is no meaning to a body that is outside of those meaning networks, that transcends them. Joanne noted that these processes are material, which was implicitly my point, but there is a difference between claiming that meaning is created in material social processes and there is something inherently meaningful in matter itself. To point to an analogy, when I point to the fact that race is a social construction, I am frequently told that race is 'real'. After all, one can see it quite clearly, not realizing that the have moved from 'matter' to structures of social signification.
So lets move to your question, what about the gas chamber? I would argue that a gas chamber (which is a stronger analogue, since the Nazi Holocaust is a historical process and not a thing) doesn't mean anything in and of itself. To make it 'speak', one needs a set of social processes, witnesses, experts, documents, etc. Nothing about it 'transcends' those structures of social reality. This is dealt with in early documentary evidence, in particular Night and Fog and Shoah. Those social structures are real, but their reality exists only in material social processes.
The Nazi Holocaust is a social construction in a double sense. In the first sense, it was planned and executed within a set of social discursive networks of an authoritarian state. In the second sense, the 'meaning' of that historical process is constructed socially. Moreover, that 'meaning' is deeply contestatory and contradictory. The reason why a book like the Dialectic of Enlightenment is so devastory is that it shows how the horror of the Nazi Holocaust was socially constructed out of the principles of the enlightenment. (Arendt's Totalitarianism adds to this as well) It seems that invoking some sort of transcendence is a form of avoidance.
This is obviously not a formal essay, and I suspect that there are some problems with it, but hopefully it works out why social construction is not in fact a form of relativism.
robert wood
P.S. In regards to Charles questions, which I will try to get to. There is a fairly long and distinguished tradition of feminist critiques of Levi-Strauss, of which Butler is only continuing. I would suggest Gayle Rubin's Traffic of Women to begin.