Gregor Clegane appeared in what novel that  is the fifth novel in the series, "A song of Ice and Fire".
Gregor Clegane:  He subsequently appeared in "A Clash of Kings" (1998), "A Storm of Swords" (2000) and in "A Dance with Dragons" (2011). A Dance with Dragons: A Dance with Dragons is the fifth of seven planned novels in the epic fantasy series "A Song of Ice and Fire" by American author George R. R. Martin.
A Dance with Dragons