bigfrog,
I can't help you on the amount of gypsum, but can suggest an alternative option that has worked well for me. A soil test would be preferable first to determine the amount of lime needed, but its probably on the order of 5 tons.
Just call your local feed store/coop that handles pasture liming and order 5 tons and have the spreader truck put as much as possible in the pond itself and in the surrounding immediate drainage area. Your water clearity problems will be fixed for many years, minimum expense, and no real hard work involved. Its just another option to consider.
