I too had a code and it was
listed under the game tab in my account. Nevertheless, a few weeks ago I checked for it and has been removed from there. Therefore you should contact support so they can send you your key.
I contacted GOG support to ask MY ADQUIRED key back. I remarked I bought the game BEFORE key removal, and that I HAD that key. They completely ignored that fact and some Natalia PR lady just told me that keys were removed by dev's decision and to f*ck off (with slightly kinder words).
Well, after reading your post I read more posts and it seems that the game is finished. Long history short, GOG version and Steam version were different, but now the creator of the game (Francisco) has finished the game and some minor details aside, he claims that the use of serials is no longer needed because now the game is DRM-Free in all the stores, not only GOG. I think this is the reason you have been denied your code, simply because the creator of the game has put them to disuse.
If you want more detailed information, you should check this forum's latest threads (this comes to mind, for example
and the website. Nevertheless, I would advise you to contact the creator of the game. He has an account here, "franfistro" (the OP of the linked post), so you can read exactly his words, and then contact him (I suppose that it is better if you contact him directly via his website
. It is said that he is very nice, but I couldn't confirm as I have never spoken to him. I don't know if he will give you a steam key, but if you can prove to him you purchasing the game before the deadline, you might
If you don't get it don't be too worried. I activated my steam key, but don't find the version any better than GOG's. And the post I linked before shows that we will get multiplayer soon.
Best wishes and I hope you have better luck with fran!
EDIT: It seems that you ninja'ed me while I was typing. Glad to see you got your key!