It's perfectly possible this "riddle" did its job perfectly well, by determining you weren't a good fit for them, and all it did cost them was a single email.

It's perfectly possible this "riddle" did its job perfectly well, by determining you weren't a good fit for them, and all it did cost them was a single email. I would have done it, because it seems fun. And solving it proves that you know how to find libraries to do external stuff (reading/writing gifs. Knowing how to use basic ImageMagick or equivalent is a useful skill), and that you can solve some algorithmic task, or at least have an interest in it (no idea how hard reversing game of life is, or if this specific problem is bruteforcable, but if they ask it, I guess there's a solution).