Question: Hafþór Júlíus Björnsson plays an HBO character who first appears in book form in what year?
Sentence (from "Gregor Clegane"): Gregor Clegane, nicknamed "The Mountain That Rides" or simply "The Mountain", is a fictional character in the "A Song of Ice and Fire" series of fantasy novels by American author George R. R. Martin, and its television adaptation "Game of Thrones".
Relevant evidence: No