Try going to the Uplay store, putting the item in your cart and entering the key as a "promo code". No promises but that was how you activated the Far Cry 3 keys from the "Never Settle" AMD promotion.
So is it possible to download an ubisoft game outside of the GG download link?
I traded for an unbisoft key, so I can activate the game if I can download it.
Unfortunately I can't download the game since it wasn't initially purchased on my GG account...
Alternately, try the method shown here