you have to trade
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haunter has to be traded to evlove haunter evolve into gengar in level 42
They evolve naturally, ie graveler at level 40.
Catch a Gastly (either in Burned Tower in Ecruteak City or in Sprout Tower in Violet City) and evolve it to a Haunter (it evolves at level 25). When you have a Haunter, just trade it and it will evolve into Gengar.
The only way is to trade it. You'll need to trade for a Haunter or Gengar if you want to own one.
Wooper will evolve at Level 20.