Yes
you trade haunter with someone
he evolves by trading him
None!
Get a Haunter and teach it Scary Face. Give Haunter a Quick Claw and take him to Mersprit.
Haunter evolves into Gengar by trading. Since the GTS is not functional in Pokemon Diamond and Pearl anymore, you will need to ask a friend, who has a copy, to do a trade-back with your Haunter.
Gahstly will evolve at lvl25 to Haunter then trade haunter to evolve to a Gengar.
you can trade it at the guy at snowpoint city
Gastly evolves to Haunter at level 25. Then, Haunter evolves into Gengar by being traded.
You have to trade to another game to make it evolve.
you trade him. and he will evolve into gengar. you trade him. and he will evolve into gengar.
Get a Haunter and trade it with a friend in the Union Room, then it will evolve into Gengar if it doesn't have an everstone.
A good team for Pokemon diamond or pearl is probably Empoleon, Staraptor, Roserade or, Steelix, Machamp, and Haunter or Gengar.