This about the merch store discount code? https://www.gog.com/forum/general/buy_games_to_receive_a_discount_on_cool_merch_from_the_cd_projekt_red_gear_store_bc684

Unique keys would only be for games where it might be relevant. Mine with Neverwinter nights won't let multiple instances of the same key be active. Otherwise games should be pre-mastered with a working key for single-player so said CD keys are few and rarely needed.

You MIGHT have a key on the BOX of the game if you got a physical copy, and then could redeem your copy here like for The Witcher 3, but if you bought the digital copy that seems unlikely. You might ask one of the blues for clarification.

If you redeemed your game through a CD key, it'll appear in the receipt of your activation. To see that, go to https://www.gog.com/account/settings/orders then drag your mouse over the order, click in the arrow that appears besides the order ID and click in "view receipt". Your key should appear in the page that just opened.

Now, if you instead thought here is a store that sells Steam keys, then you are mistaken. Games bought here cannot be activated elsewhere.

The Witcher 3 doesn't need a CD key as far as I'm aware. Is it asking for one?

THIS IS THE RIGHT THING TO DO!!!!