First, you need to catch a Ghastly, which can be found in the sprout tower or caves at night. Evolve it into a Haunter and then trade it. At that it will evolve into Gengar which you need to have it traded back to you.
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.
To get a Gengar, you must trade a Haunter and it will then evolve into a Gengar.
Gengar is a Ghost-Poison type.
gengar can defeat any Pokemon same with any other Pokemon
Psychic Bryce has a gengar in Pokemon platinum.