Question: are sir gregor and the mountain the same person?
Answer: Ser Gregor Clegane is the Knight of Clegane's Keep and the head of House Clegane, a landed knight and bannerman to House Lannister of Casterly Rock. Gregor is a freakishly large man and for this he is often called the Mountain That Rides or simply the Mountain. His soldiers are known as the Mountain's men.