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Sorry for the delay... I went to take a shower... As for the emulator thing, you only have to open one and open two games in it. You may not have the right emulator. PM I'll give you some french websites that found when I lived in france. Its not really illegal there so they have HUGE selection of emulators and games... and yes I'm french which is why my english isn't very good.
To get Gengar in a ROM, if you have the Visual Boy Advance Emulator, then there is a GameShark Feature, try using it or try buying the actual Cartridge & buy a real GS & use it to get Gengar.