"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."
I do believe Moonlight is a bit better but Steam Streaming has been reliable and high enough quality for me not to care enough to use Moonlight most of the time.