as i type this my eyes are welling up with tears of laughter....
sheet rock subwoofer boxes.....rofl
i don't think drywall is very resonant, so there is that one good quality, however unless you can find 2" thick sheet rock that doesn't crumble like stale bread, i don't think it will work very well. the dust that stuff makes would probably negatively affect your subwoofer's moving parts as well.
for my sake, and for the sake of everyone else that read this post, please tell us you were kidding....