Well the way I got my NES roms going on my card was is that I used Pocketnes to make a ".gba" file than I flashed it to my card as a gameboy advance rom. I'm not sure if there is any other way to do so but that's the way I did it. I don't have any links but if you google "pocketnes" I'm sure you'll find it.
That download comes with everything you should need to get it working. Just transfer the ds.gba file created over to your G6 using Explorer and then go through the MyCard menu on the G6 to start the game(s).