GPG - abbreviation for 'grain per gallon'.
Grain - (gr.) a unit of weight equal to 1/7000th of a pound, or 0.0648 gram.
Grain Per Gallon - (gpg) a common basis for reporting water analyses in the U.S. and Canadaone grain per U.S. gallons equals 17.12 milligrams per liter or parts per million. one grain per British gallon equals 14.3 milligrams per liter or parts per million.
Gram - (g) the basic unit of weight (mass) of the metric system, originally intended to be the weight of 1 cubic centimeter of water .
Greensand - a natural mineral, primarily composed of complex silicates, which possess ion exchange properties.