The gypsum in sheetrock can help clear up a muddy pond, but is not very effective at stopping pond leaks. I would not use sheetrock, myself, because of other materials present in sheetrock in addition to the gypsum.
Bentonite can be very effective when applied in large amounts to an empty pond bottom and mixed into the bottom soil....but when applied through the water column, it is significantly less effective. In fact, it is rarely successful when applied to a full pond. The folks who sell bentonite here in Texas will tell you the same thing.
Pond leaks can be very difficult and sometimes very expensive to fix. Good clay, when available on your property, remains the most effective, economical method of lining ponds.