Why is my fresh tofu fa (douhua) sour?
Quality of the beans aside, it's likely that you soaked your beans for too long and they have started to ferment. Gypsum is basically tasteless so it should not have an effect on the taste. 

When soaking soybeans for soymilk you will want to soak them for no longer than six hours (four in the summer or areas with high humidity). If you soak them for too long, they will start to ferment (thus producing the sour taste), which is likely to be the cause of the sour taste. If you may also want to consider soaking them in the refrigerator to prevent fermentation. 

Consider tasting the soymilk batch you produce to ensure that the beans you used has not started to ferment.