GH

General Hardness (GH) is the measure of Calcium (Ca2+) and Magnesium (Mg2+) ion concentrations dissolved in water. These minerals are present in municipal, well, and bottled spring water. The level of general hardness in tap and bottled water depends on the source of the water and the treatment process it has undergone. Hard water (=200 ppm) is high in calcium and magnesium, while soft water (50-100 ppm) is low in these minerals. A GH level between 100-150 is recommended. One drop of the testing solution is equal to 17.5 ppm. 

(Ref. 8) 

(Ref. 10), (Ref. 11)