@hightor,
problem with gypsum is that, unless the soil is kpt aerobic, yull have H2S gas smells being created from the anaerobic decomp of the hydratd Ca SO4. The SO4 turn to H2S and the whole damn area smells like a MAine Papermill. Weve tried to get the commercial mushroom composters to go to some other "slickenside" product but gyp board is cheap and easily added to the mix.
The German aerobic method keeps the smell down but theres still a bunch of stank cause the mushroom guys dont follow the recipes.
I know its only a home compost thing but over a year, if not cared and kept aerobic , the compost will really stink like sulfitic cabbage.
I love Maine, but if only theyd change their damn pulping methods it wouldnt stink as much. You can smell it all the way out on Monhegan