i dont think you can i suggest you get gastly and evolve it
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If you have Fire Red in your DS, go to the Old Chateu.
the old chateu is only in Pokemon diamond pearl and platinum versions
A person is able to get Gengar in Pokémon Diamond in the Old Chateau room. They may also trade with a player to get this Pokémon.
Finding a Gengar in pretty simple in you insert any GBA game into your DS and go into The Old Chateau near Eterna City and go into the room with the eyes on the wall.
Gengar is a powerful ghost type of Pokemon. In Pokemon Platinum it is possible to capture one by going to the Old Chateau.