"Grains per gallon" (GPG) refers to the level of minerals in the water, which determines its hardness. It is crucial to know the GPG value in your water to select an appropriate descaler. You can consult a plumber or contact your local water department to determine the GPG level. Different descalers are designed to handle varying GPG ranges, such as up to 15 GPG or even higher. It's important to choose a descaler that can effectively handle the hardness of your water to achieve the desired descaling results.