Gypsum (calcium sulfate) adds permanent hardness (calcium ions) to brewing water. Calcium Sulphate causes permanent hardness because it doesn't get removed by boiling. It is believed that sulphate suppresses harshness and astringent flavours which therefore allows brewers to use large amounts of hops without extracting undesirable harshness from other components of the hop. Calcium Sulphate shouldn't be overused though, as it can create a harsh taste and impact on other aspects of the brew.
