you trade him. and he will evolve into gengar. you trade him. and he will evolve into gengar.
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Get a Haunter and trade it with a friend in the Union Room, then it will evolve into Gengar if it doesn't have an everstone.
You trade Haunter to another Diamond, Pearl, or Platinum version and it will evolve into a Gengar. You will need to trade back to your original Diamond game to get the Gengar. Hope I helped, bye
You have to trade to another game to make it evolve.
you can trade it at the guy at snowpoint city
Gastly evolves to Haunter at level 25. Then, Haunter evolves into Gengar by being traded.