“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.”
Geforce Now is a cloud streaming app, you can already use Nvidia Gamestream on deck with moonlight.
GameStream is dead. You have to use Sunshine with Moonlight now.