If the gas coming off is sulfur trioxide, it will form sulfuric acid just by absorbing moisture from the drywall itself, as gypsum, even if apparently bone dry, always contains water in its crystals. This would probably make the sheetrock crumble as it destroys the gypsum crystals. If its sulfur dioxide gas, that can convert to the trioxide in the presence of air and a catalyst, such as metals, salts, oxides, organic materials or sunlight. Both gasses are toxic. Once the acid is formed, it will strongly absorb water from any source and hold onto it tenaciously. IMHO this acid will not be removed from a house just by ventilation, for in a damp climate that could just make matters worse. In fact, H2SO4 corrodes some metals better when dilute. The good news is that it is killed by copper and cement, so once all the wiring, concrete and plumbing have vanished.....
