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.
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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.
You can find one on Route 44.
If you can trade Pokemon from crystal version to gold version my guess is yes
The only way is to trade it. You'll need to trade for a Haunter or Gengar if you want to own one.
A fake version of Pokemon gold