If held by a Gallade, the Galladite allows it to Mega Evolve into Mega Gallade if its Trainer has a Key Stone. A Trainer can only Mega Evolve one Pokémon per battle (or only have one Mega-Evolved Pokémon at any given momentZA).
As a Mega Stone, it is unaffected by item-negating effects (such as Embargo and Klutz). Item-manipulating effects (such as Trick and Fling) cannot give or remove a Galladite from Gallade .
Outside of battle
If held by a Gallade, the Galladite allows it to Mega Evolve into Mega Gallade when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Gallade will remain a Mega Gallade until the Talent Round ends.
In PS616, the Meteor Villageelder offered Sapphire a Gardevoirite and Galladite, commenting on how her Gallade and Ruby's Gardevoir are a match. Ruby and Sapphire later used these Mega Stones to Mega Evolve Rara and Kirly during a brief battle against each other.
