A GPG is equivalent to 17 ppm. They will send you sample bottles for the number of samples you want tested. It includes hardness, pH, bacteria, nitrate, metals etc. Test results will be sent in about two weeks after receiving your sample. The significance of hard water to pesticide efficacy is not quite clear.
Hard water seems to be more of an issue with herbicides than with insecticides and fungicides.
Hard water is definitely an important consideration for Apogee application. Choice and Quest do same thing as ammonium sulfate, they attach to ions in hard water and prevent them from binding with the spray material. You will not be dealing with Apogee until next spring at the earliest, but since we are on the subject, the recommended rates for a hard water Apogee application are 8 ozs. of Quest or 4 ozs. Thanks to Brian McLeary of United Agri Products and Gar Thomas of BASF for help gathering this information. The Highmoor Farm Field Tour and Maine State Pomological Society summer meeting is on Thursday, July 20.