I don't know if you can get it elsewhere. There is not standard amount. it has to do with what you are trying to accomplish combined with what you are starting with. with extract that's particularly problematic because you don't know what the maltster already put in the extract. if you begin with distilled water then that at least is a blank slate.
best thing to do it brew your recipe and taste it. does it seem like it would benefit from a more crisp, dry seeming finish? maybe a bit more pop in the hops? then dissolve a few grams of gypsum in an ounce of beer and use that to dose another sample until it tastes the way you want it to. you can then extrapolate how much you would want to add to the whole batch to get the same effect.
you can then include that in your recipe and the next time you brew it, assuming everything else is the same, you can add it in the boil kettle.