I have both "Frozen Synapse" and "Frozen Cortex" at Steam, obtained via a Humble Bundle ("Humble Weekly Bundle - Make Your Move!"). Currently they are available at the GOG Connect program. But they don't show up as games that I can add to my GOG library.

When I activated the keys at Steam, in addition to the game's name there was the appendix "Thirdparty Retail". May it have something to do with that?

I can't activate it either. It shows "Not found in your Steam library."

I fixed my problem after posting on General Discussion. My issue was that my Steam profile was not public (only the inventory was).

Here's the thread: https://www.gog.com/forum/general/cant_redeem_frozen_synapse_and_witchers_adventure_game_from_steam

My Steam profile is public. It seemed to have been a fault of GOG however, since I could add the games to my library shortly after my post.