Then use a minor amount of liquid soap or detergent in the water AFTER you apply gypsum. A 30 minute soak right now is excessive. It can result in wasteful runoff.
A 30 minute soak right after applying gypsum will wash away the gypsum. The initial wetting down should be less than a 5 minute sprinkle, just enough to make the gypsum damp. The next day, a 5-10 minute sprinkle will start the gypsum dissolving; do not sprinkle long enough to puddle or start rinsing the gypsum away. (Sprinkling instead of flooding will hasten the dissolving of the gypsum because of the force of the water landing on it.) Then every third day, repeat the dissolving sprinkle.