Explanation：
GPG is the unit of water hardness.
1GPG means 1 grain of hardness ions (calcium and magnesium ions) in 1 gallon of water.
According to the American WQA (Water Quality Association) standard, the hardness of water is divided into 6 levels:
(1) 10~0.5 GPG is soft water
(2) 0.5~3.5 GPG is Microhard
(3) 3.5~7.0 GPG is medium hard
(4) 7~10.5 GPG is hard water
(5) 10.5~14.0 GPG is very hard
(6) Above 14.0 GPG is extremely hard