That is similar to a question I asked about using Calcium Chloride from a pool store for brewing. The general consensus from many here on HBT was not a good idea because it would not be food grade. Probably same applies to using gypsum from drywall.
More to the point is that the gypsum in wall board may well contains things that make for better wall board. I don't know what those might be but I once thought to try plaster of Paris when the local pro bought the home brew shop out of gypsum only to find that there were things in it other than CaSO4.