As of 5/2/15, Gateway can only be used on a 3DS or New3DS console on firmware 9.2 or lower. If the firmware has been updated to 9.3 or later, as of now, it is incompatible.
If your 3DS or New3DS does have firmware 9.2 or lower, you may use the appropriate exploit for your firmware version to run the Gateway card. Depending on whether you are on a 3DS or a New3DS, you may need extra materials to run the Gateway exploit (specifically for the New3DS).
One of the key features of the Gateway is the use of what is called EmuNAND. This, in simple terms, is a copy of your firmware that you can run and update (only update when the Gateway team says its okay though), allowing you to have full compatibility with any games and features that might need the newer firmware. Since this is a copy of your system's firmware, while it isn't bullet-proof, EmuNAND is also a nice shield, helping to prevent bricking and accidental updating of your real firmware.