If held by a Gallade, allows it to Mega Evolve into Mega Gallade if its Trainer has a Key Stone. Each Trainer can only Mega Evolve one Pokémon per battle.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot give Gallade a Galladite or remove a Galladite from Gallade.
Outside of battle
If held by a Gallade, allows it to Mega Evolve into Mega Gallade when it performs a Spectacular Talent during a Pokémon Contest Spectacular while 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.