Interesting. Plancha refers to a metal, not stone surface. But I'm not sure all of the subtleties matter. For instance, Wikipedia says a plancha is a flattop grill, that being different from a griddle by being heated radially and not with a heat source lined up straight (such as a gasser's tube would do.)
That said, none of our regular griddles heated on a gas stove would be termed flattop grills ...