One of gypsum board's properties is that it allows water vapor to pass through-it has a permeance of 24 to 35 perms. Contrast that with a vapor retarder, which, by definition, has a permeance of one perm. One of the physical properties of water vapor is that it will move from areas of high concentration to areas of lower concentration; or stated differently, it will migrate from a warm, humid environment to a cool, dry environment. Additionally, warm air will hold more water vapor than cool air; so as moisture-laden warm air cools, it quickly loses its ability to support the water vapor and deposits it as condensation on any relatively cooler surface over which it passes.
Gypsum board is occasionally mistakenly identified as the culprit when water vapor has been found to have condensed and collected in a wall cavity between the interior wall surface and the exterior sheathing; some builders have even asserted that evaporation from drying joint compound is sufficient to result in condensation in wall cavities-more on that theme later. But it is not the role of gypsum board to prevent water vapor from accumulating in a wall cavity-preventing the movement of water vapor is the role of a vapor retarder (although gypsum board can be finished or treated with a coating system or an impermeable laminate, like aluminum, so that it will serve as a vapor retarder).