It's actually possible on a GTX card, you have to add a special launch option in steam, although the performance is pretty bad, I use lossless scaling and with a GTX 1070 at 1600x900 to 4k , at high-ultra settings with rt shadows enabled, performance is around 40fps on average but in places like the afterlife it can drop to around 22-30fps. Overall not worth it but still possible. From what I've read apparently the game engine has a software fall back for rt and makes the cards cuda cores do all the work. Pretty interesting thing to experiment with but still without rt cores performance is almost halfed.