Things You'll Need
Instructions
Catch a Haunter on Route 8 at night and assign it to your traveling party of Pokemon.
Start up "Pokemon Silver" or "Gold" on the second Game Boy and connect the two with the Link Cable.
Go to the nearest Pokemon Center in both games and talk to the receptionist at the Cable Club Trade Center.
Go into the Trade Center in either game and press "A" while facing the table in the middle of the room. You will now be able to see data from both Pokemon games.
Select Haunter and choose "Trade." Confirm that you want to trade Haunter and it will be sent to the other game.
Leave the Pokemon Center in the game which now contains Haunter. It will automatically evolve into Gengar. Return to the Trade Center and repeat the process to send Gengar back to the first game.