You can't catch it using Poke Balls, 'cause it is'nt in Pokemon Emerald... But you can trade with other players, of other versions of the game...
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How do you get a gengar in emerald
Trade it from from Firered or Leafgreen. Or get a gameshark and enter the CORRECT code.
yes into a haunter and the trade it to evolve into a gengar
Wild Gengar can appear in the Old Chateau if you have Ruby, Sapphire, Emerald, FireRed, or LeafGreen in the GBA slot. In this case, it has a 4% chance of appearing.