“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.”
No, it isn’t.
EDIT: After reading other comments I realize I mistook GeForce Now for GeForce Experience. While I still disagree that SD/Linux is “crying out for it” I actually think bringing GeForce Now to Linux would be a good move.