the only thing similar in supernatural and the witcher is supernatural have two clear protagonist, the witcher one, but taht's close. And the characters are hunters who seek in some minimal degree inside their jobs, justice, accepting the grays of life. That's is only SOME of the argument. EVERYTHING ELSE IS DIFFERENT, and how the story is designed, how the world is, how the monsters are, how is shown this story and world, all the story except for the job of hunting things, is obviously close to game of thrones and even better designed to me, because is based on books similar to game of thrones but better in my opinion.
Sorry but nothing to do with Supernatural.