What did you choose for your other copy of the game? AFAIK there is only one DLC included in the tiers with 2 copies of the game.
When I redeemed my code it took a day or two for the DLC to show up, but at the time GOG was in the process of getting the Mac version up and adding the DLC. I'm not sure if a delay now is out of the ordinary, but if you are not in a hurry to play the GOG version with the DLC items, you could wait a couple days to see if it shorts itself out.
I'd check with GOG first to see if there was a problem redeeming the code, and then check with Larian if they say it is just a standard edition code.
