How much gypsum is required to coagulate one litre/gallon of soy milk
I have The Book of Tofu by William Shurtleff, and the more recent Asian Tofu by Andrea Nguyen. 

Shurtleff recommends 2 tsp of gypsum for 7 1/2 to 8 cups of soy milk.
Nguyen recommends 1 1/2 tsp for 8 cups of soy milk.

(These recipes are for about 1/2 gallon of soy milk. There are 16 cups in a gallon)

I can't verify with personal experience, as I use Epsom salts (magnesium sulfate) as a coagulent, but have had success with both books.  So, I am sure they would both work.