"There is not a native app on Steam deck today," said Andrew Fear, GFN boss, back in January. "Use a Chromium browser to make it work. I would say that both Nvidia and Valve, I think we're both interested in making [GeForce Now on Steam Deck] better. But we don't have any announcements on a native app coming to Steam."
It’s not a subscription plan with games, it’s a cloud service running the (supported, stuff about licensing) games you already own on Steam.
Ah sorry, my misunderstanding. Though, isn’t that just built into Steam anyway?
You know what, I’m going to read up on it myself, so as to be less wrong in future!