Where is Galladite in Pokemon X/Y?
Pokemon X and Y do not have access to the full set of Mega Stones - some were introduced in Pokemon Omega Ruby and Alpha Sapphire.
Galladite is one such stone: it was introduced in Pokémon Omega Ruby and Alpha Sapphire. It cannot be found in - or traded to - Pokemon X/Y.
Source: Bulbapedia - Galladite