He asked for a card that works to play original DS games. Most of those were blocked except for the DSTWO and certain R4 cards.
However, hacking 3DS mode will let you install things that are used to unblock DS flash cards, so every DS card will work.
Gateway works by using an exploit. Exactly which exploit it uses depends on your system and firmware version. New 3DS requires a rare game (Cubic Ninja or Ocarina of Time, and Ocarina of Time also requires a way to get a hacked savefile into the cart.) Original 3DS and 3DS XL use other exploits. Gateway only works on firmware up to 9.2, although it can load an emulated version of later firmware.
Sky3DS just exactly simulates a real cart. So it works on any firmware, but it can't play homebrew, out of region games, 3ds eshop games, or run other hacks like the one you need to get blocked DS cards to work.
There are now custom firmwares available which have some of the functionality of Gateway without having to buy anything. You still need to have firmware 9.2 or less and you still need the rare game if you are on a New 3DS.