GeForce Now can be used with Steam quite seamlessly. In fact, GeForce Now's value is to play the games you already own on various platforms on their hardware. They're not trying to be another games store.
Now, they put you in a weird virtual zone where your library is restricted to only the game you are currently playing. And if that game is not allowed by the publisher, you can't play it. So I can't play Slime Rancher. I can't play Shadows of Mordor. And it seems like over time there is less and less there.
It sucks, because it was basically my dream gaming service. Kind of like Google Play Music was my dream music service. :(