When you trade him/her like in all other generations.
You'll need to trade for a Haunter in order for it to evolve into Gengar.
in any Pokemon you evolve haunter by trading
Gahstly will evolve at lvl25 to Haunter then trade haunter to evolve to a Gengar.
Gastly evolves into Haunter at Lv25.
You have to trade it and it evolves.
Haunter evolves into gengar once you trade him.
Haunter can only be evolved into Gengar via Trading (such is the case in all Pokemon titles that include Haunter, not just Silver).
You have to trade it to an other game for it to evovle.
You have to trade the Pokemon to evolve it.
You will need to level up Haunter to level 50 to evolve it in Pokemon Resolute.
Actually, Haunter does evolve. Its evolves into Gengar when you trade it.
Trade Haunter with any Pokemon to make it evolve!
In order to evolve Haunter, u have to trade and it will evolve into a Gengar.
Yes. The pokemon is Haunter. In order to evolve Haunter you will have to trade him with a friend and he will evolve. Just trade back to get Gengar.
You'll need to trade for a Haunter in order for it to evolve into Gengar.
You trade haunter to get gengar.
Haunter evolves when traded