These two terms can be a bit confusing, but what you should know is that they are different and can never use interchangeably. A gravestone is a slab used to cover the grave and has the name of the deceased with the year he or she was born and the date of his demise.
The gravestones are mostly found at the gravesites, and most people might have come across one; what confuses people is because a modern gravestone is used with tombstone and a headstone.
On the other hand, a headstone, also known as the cornerstone, is a square stone placed at the head of a grave. The information in a headstone, the names of the deceased, date of birth, and that of demise.
The difference between headstones and gravestone is slight, but it matters a lot; confusing them is normal, but now you know. The information written on both of them is the only similarity they have.