You can pre order the game via gamestops digial downloads. Once you place your order with them on their digital downloads site, click on my account, then games purchased. click "view" for that order you placed and it lists your cd key there. Once you have your cd key, simply go to steam and then click "games" and "activate a product on steam."
It then ties the game to your steam account and you can go ahead and pre load it as though you bought it on steam. You won't get your pre order items from gamestop until tomorrow (they are emailed) but this is a way that you can play the game tonight with the Steam pre orders while getting the best pre order pack also since the binocs you can buy in the game.
Already did this myself.