I'm not liking these exclusives for pre-order places, but what can you do right now? In any case, if you pre-order Batman: Arkham Knight from Gamestop, you'll get to play as Red Hood in a story pack. You'll also get the Harley Quinn challenge pack that was announced a while ago as well.
While I love as much Arkham Knight content as possible, this practice of only making it available from certain retailers isn't really giving me good vibes. I hope that they do release it later on down the road for those that have ordered from some place else to play. I'm guessing that we'll have to wait until the Game of the Year edition for that, but a timed exclusive to Gamestop is what I am hoping for.