“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.”
Yes but others are talking about steam streaming, which is also my comment here.
It’s tangential but worth calling out in this discussion, especially as you can do remote play with moonlight (haven’t personally tried it though)