You must have any Third Generation Pokemon game cartridge in your bottom slot of your DS then when you have the national dex, then you go to the Old Chateau in the Eterna Forest, Go to the second floor and enter the second to last room on your right. The room with the eye is where you will find Gengar but it will take a little while.
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Gengar can only be obtained by trading a Haunter.
You'll need to trade for a Haunter or a Gengar in the GTS or from someone.
All if you have a gameshark
NO,you will have to trade it to evolve it.
you can't catch a gengar. you catch a haunter then trade it so it evolves. make sure its not holding an everstone. ;)