You'll have to trade it from Fire red or Leaf green version
Although its 1st and 2nd evolutionary forms arecatchable; Gengar, unfortunately, are not found in the wild. You can only get them by trading a Haunter from Fire Red or Leaf Green, or through a GameShark/Action Replay code.
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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.
How do you get a gengar in emerald
You can't find gengar in the wild, you have to get a haunter and trade it.
Trade it from from Firered or Leafgreen. Or get a gameshark and enter the CORRECT code.