SteamOS is Linux based but I doubt it's going to be anything even Stallman would call GNU/Linux.
Everything I've seen makes this sound like it's more aimed at being a 'Console which runs PC games' than a normal computer. I'd expect it to load into a 'Big Picture' mode Steam client, and allow the user to launch their games and specially-modified applications from that which could well run as overlays like the existing Steam browser. Whether this machine even needs a command line is debatable, it shouldn't need GCC (I'd expect a fully binary-based OS) or a full-featured window and compositing desktop like Gnome.
It has just been discovered that research causes cancer in rats.