Gengar is never a wild Pokémon and therefore is not catchable. The only way you can obtain it is to trade a Haunter.
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You cnnot catch it in Pokemon fire red ,but you can catch it in Pokemon Diamond and Pearl.If you want it to be in your fire red,you will have to migrate it from your Diamond/pearl to your fire red.
You can get gastly and haunter in lavender town. Trade a haunter to a friend to make it evolve into gengar.
If you have Fire Red in your DS, go to the Old Chateu.
You can only catch them in Pokemon fire red.
You can't catch it on Pokemon Fire Red. You have to trade it from Leaf Green.