True 3DS-mode flashcards are not region free, they faithfully emulate an original game, and the console may or may not accept the game depending on any other mods done to it
The Gateway "flashcard" requires their own (outdated) CFW to work, and said CFW includes region free (so do most ones that don't require additional hardware)
DS-mode flashcards are region free as always
DSi-mode flashcards (er, singular) are region locked as always, not like it's a smart idea to buy one outside of bragging rights