How do you get a gallade on Pokemon diamond?

You can't get a Gallade in Pokémon Diamond.

Here's why:

* Gallade is a Gen IV Pokémon: It was introduced in Pokémon Diamond and Pearl's sequels, Pokémon Platinum and HeartGold/SoulSilver.

* Evolution: Gallade evolves from Gardevoir, which in turn evolves from Kirlia. Kirlia is not found in Pokémon Diamond, only in Pokémon Pearl.

How to get Gallade in other games:

* Pokémon Platinum: You can find Kirlia in the Grand Underground.

* Pokémon HeartGold/SoulSilver: You can find Kirlia in the wild or obtain it from the National Park.

Note: To evolve a Kirlia into a Gallade, it must be a male and know the move "Synchronoise" before leveling up.