Is it true that you can encounter a Gengar while having a 3rd Generation game cartridge in your DS?
Yep, You do encounter Gengar and haunter when you have a third gen game in your DS.
Is it true that you can encounter a Gengar while having a 3rd Generation game cartridge in your DS?
So.
Do you need R/S/E or Firered, Leafgreen in order to aquire this?
Yes. You can get Gengar. Please, try searching. ^^;Duel-Slot Transfer is when you have a game in the second slot or GBA slot of the DS. For example, put in a Ruby, Sapphire, or Emerald game in the GBA slot of the DS. Go to Pal Park and transfer 6 pokemon from the GBA game to the DS, and catch them all, and save the game. Keep the game in the slot and return to the Old Chateau. In the upstairs hallway or in the room with the painting with the red eyes, You'll be able to catch Haunters between levels 20-30, and Gengars between levels 25-40. I hope this helped answered your question.
Many of the Pokemon Games, recently Platinum have many scripts that are unused in them. I found these scripts out. Like the Odd Keystone script. Go to Twinleaf Town, surf on the little lake near your house in the game, and try talking the third or second tree on left side of the lake. It should say, " [Player's Name Here] has obtained a Odd Keystone! "
Another one is on the Cycling Road, you can see an odd tree colouring in the far back right of the screen on the mountain. ( Action Replay used on Pearl. )
There's many more to explore! :D
( Sorry for the offtopic chat. I wanted to explain that. )