And here it is, the gameplay, rendering and many other features are pretty much almost ready. I believe that at this point the game actually satisfies everything the challenge requests. Including, somewhat dark but still complete main menu:

Fig. 01 - Main menu with fonts and some effects

And the rendering was finally changed to something better than colored preview ... so naturally the game is fully path traced (it is noisy, it is slow, but it is ... AWESOME). I will probably provide also non path traced build as playing it in fullHD is pretty much not possible (doing few samples per pixel is a bit of overkill, even for such small scene).

Fig. 02 - DOOM has brought us ray casting and some interesting new effects, GROOM follows the spirit of DOOM and implements fully path traced rendering

And yes, I've actually named it GROOM (no idea why - it just sounded well to me). So, there will be few more updates improving the playable area slightly, but generally this is it - I know it is small, but pulling off GPU ray tracer for dynamic scenes, that isn't too slow, isn't as easy as it sounds. It was a nice ride - and once release (which will happen within next 24 hours), you can try it yourself to see, how path tracing really is slow.