I am sure the Moisture Reading would range from near true to extremely high. Gypsum is naturally encountered as a suite of minerals ranging from Hydrate and as an Anhydrate. The crystal structure is variable and always has the ability to include water as part of the chemistry. It is the amount of water which determines what the exact mineral is.

I am not sure what the effect on the Density Reading would be other than probably a high wet weight. My dealings with gypsum and some recycled concrete aggregates have required many samples taken for laboratory drying. Remember, the drying temperature is only slightly higher than a hot summer day.