Gray iron or grey iron was the original "cast iron". It is an alloy of carbon, silicon, and iron, containing from 1.7 to 4.5% C and 1 to 3% Si. It is relatively easy and inexpensive to make. Compared to the more modern engineered irons, gray iron has a lower tensile strength and lower ductility.
In other words, it will fail more easily, and its mode of failure will be sudden fracture (it will not bend). It is used for housings where tensile strength is non-critical, such as engine blocks, pump housings, valve bodies, electrical boxes and decorative castings.