First the chemistry
gray iron has more of the following elements
-Sulfur (bonded with pyrite)
a dash of titanium
Polished up it looks like this
In a foundry gray iron gets poured much faster than the ductile iron because it can be melted faster and it needs less additions before it gets poured into a cast
As you can clearly see there are no nodules present. The addition of magnesium right before the iron gets poured forces the iron to form into nodules. This makes the iron much stronger in tensile and hardness aspects.
Gray iron is much better for absorbing and redistributing heat...so it still has plenty of uses.