WHAT IS IT, REALLY?
Most aquarists use the term "hardwater" generically to refer to water that is high in mineral content - this can mean either the water being high KH or high GH or both. It is common for "hardwater" to have both high GH and KH because sources of hardwater are areas with limestone [CaCO3] - which contributes to both GH [Ca ion] and KH [CO3 ion].
However, hardwater can be high in GH and low in KH and vice versa.
Minerals that can raise GH without raising KH are minerals that contain calcium/magnesium but do not contain carbonates : Magnesium sulphate [MgSO4], Calcium sulphate [CaSO4]
Minerals that can raise KH without raising GH are minerals that contain carbonates but do not contain calcium/magnesium: Potassium Carbonate [KCO3], Sodium bicarbonate [NaHCO3]