You can use the Pokemon modifier for action replay but I don't know the code right now.
Gengar can only be obtained by trading a Haunter.
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.
Sorry to say, but Gengar is not obtainable in the wild.
you trade him. and he will evolve into gengar. you trade him. and he will evolve into gengar.
no
you can't.you must trade
evolve it by trading it with another player
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.
You cannot get these pokemon without trading. But you can cheat by using gameshark codes.
Haunter evolves into Gengar via Trading.
Trading
in any Pokemon you evolve haunter by trading