To get Gengar you must link trade Gastly with a friend that has a 4th generation game (Diamond,Pearl,Platinum,Heartgold,Soulsilver) and then it will evolve into gengar.
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In Diamon/Pearl/Platinum/Heart Gold/Soul Silver, level 55. In LeafGreen/FireRed, level 53.
There is no NPC who you can trade a Haunter to for a Gengar in Pokemon Crystal. To get Gengar you will need to trade with another player or a copy of Pokemon Gold, Silver or Crystal.
The only way is to trade it. You'll need to trade for a Haunter or Gengar if you want to own one.
haunter has to be traded to evlove haunter evolve into gengar in level 42
To get a Gengar, you must trade a Haunter and it will then evolve into a Gengar.