Guys, I work in the gypsum board manufacturing industry and am very familiar with the composition and formulation of gypsum board. I also own a pond.
I would never dump gypsum board in my pond!
There are a couple of issues to consider. 5/8" gypsum panels aka "Type X" have quite a bit of chopped fiberglass fibers in the core. When you break a piece of the panel and hold it up to a light, you will see what looks like very fine hair. That's the fiberglass.
I don't recommend swimming in a pond with chopped fiberglass fibers. They can be a skin irritant and not good if ingested.
Other raw materials in gypsum board include inorganic foaming agents, dispersants, and setting agents. If you drank any of this stuff you'd be on the toilet for a week!
In the old days, especially where there were gypsum manufacturing plants located next to gypsum quarries, the plants would dump the waste board in the worked out pits that later filled with water. Even a blind person can tell when they are getting close to one of those pits because the smell of hydrogen sulfide gas is unmistakable. Of course, the water has a very high concentration of decomposing gypsum panels, thus the reaction. Nevertheless, gypsum is water soluable and it's chemical formula is CaSO4-2H2O.
Don't forget that for each square foot of gypsum panel that goes in the pond, there is two square ft. of sized paper that has a basis weight of nearly 40 lbs/thousand sq. ft.
If you want to put gypsum in your pond, use the material made for just that purpose. It's what we call 'landplaster' which is natural (as opposed to synthetic) gypsum rock ground to the fineness of face powder. The gypsum particle will attract a clay particle and the resulting particle is heavy enough to sink.
