Go to Victory Road, make sure you have someone with Rock Climb, and go to the first trainer on the bottom left (You'll see it) and he's a Psychic named Bryce. He'll battle you with a Gengar.
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Gengar is a powerful ghost type of Pokemon. In Pokemon Platinum it is possible to capture one by going to the Old Chateau.
Psychic Bryce has a gengar in Pokemon platinum.
If you have Fire Red in your DS, go to the Old Chateu.
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A trainer in Victory Road has a Gengar.