you can't.but thre is a way.youneed to catch a male gallade then use ....... stone.
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You must first evolve Ralts into Kirlia and then you must use a Dawn Stone to make it evolve into Gallade.
If your Ralts is male, then it goes from Ralts, Kirlia, Gardevoir, OR if you give it a dawn stone before evolving it into Gardevoir, it evolves into Gallade. If it is Female, it cannot evolve into Gallade, it will always be a Gardevoir.
Gallade is a evolved form of Kirlia (Evolved form of Ralts) but has to be a boy Kirlia. If it is, then you can evolve it with a Dawn Stone, and it will be a Gallade.
First, capture a Ralts from a Pokemon Swarm (or trade from someone else). It must be a male Ralts. Then, evolve it into a Kirlia. Then, use a dawn stone to get a Gallade.
Ralts evolves into kirlia at level 20,then gardevoir at level 30 or gallade if its male with a dawn stone