Unity is only the shell, it uses gnome underneath as the desktop environment. It's also the name of a Linux distribution that uses openbox to cause ultimate confusion.
No. Unity has nothing to do with Gnome or the Gnome project. It owes its existence because of Shuttleworth and Gnome being at odds. You
could call it a "plugin" to Compiz, which would be technically accurate, but you are completely missing the point. Unity is a complete DE, not a window manager. In fact, the roadmap is to drop Unity 2D entirely (which is written in QT, not GTK) and optimize a low end hardware solution from unity 3d. Not only that, they are optimizing Unity to run on weyland. You can't just slap Unity on top of GTK (or QT) and X and run it.
You want to act like the desktop linux world is some wild west morass of window managers, X windowing systems, file managers etc that have no cohesiveness except what is brought by some bright linux hacker, which simply isn't true. Installing all of the mainstream distros is brainless at best, and they work 99% out of the box. The ONLY thing they struggle with are apps which are so completely sandboxed (DirectX, Silverlight, .Net...notice a pattern here?) that there is no real solution for. People don't use linux because of games or because some craptastic shovelware program doesn't run natively on it that they just CAN'T live without. And because most people can't follow simple instructions on web forums. For those people seriously TDO, there is always OSX and MS. That bar is getting way down there now.
Unity is also a 3d game engine, recently ported to OpenGL and linux. It really isn't causing any confusion.
Ubuntu has been touted as the next big thing for years, if steam goes well then it could become a viable desktop operating system that is free as in beer, if not free as in spirit. It will only succeed if people spend money on games though.
You are missing the issue here as well. Its not as much that Steam now runs on linux natively, its that
Source runs on linux natively . Its that Unity runs on linux natively. Which means that these engines run on OpenGL. Which means they do not run on DirectX. Which means that MS is well and truly screwed in the game arena.
EDIT: That's probably poetic license. What it really means is that games will be optimized for both, which is a very good thing. A boy can dream, right?

Go to the Humble Indie Bundle site and see how much money linux gamers spend on old has-been PC games. The market is there.
Ubuntu make their money by also providing Amazon search results when you're searching for local content. Amazon is very toxic in the UK, because like Starbucks they avoid paying any corporation tax at all. Not exactly the kind of company you really want to be involved with. http://www.theweek.co.uk/uk-business/46200/how-does-amazon-minimise-its-tax-payments
I think I'd rather use Microsoft products :-)
Nope:
http://www.zdnet.com/canonical-performs-u-turn-over-amazon-search-results-in-ubuntu-12-10-7000004950/Even if your argument made sense, there are hundreds of other distributions with equally well integrated DEs. Fedora, Debian, CentOS, Mandriva, Suse come to mind. I even left Mint off since its close to Ubuntu.
The instant I don't have to program in C# and .net or the day I can play Fallout 4 in my linux steam client at 80+fps at 1920x1080 is the day I never have another MS partition on my home computer. Its that simple.
Disclaimer: I do run Arch and I do like mucking about with all of those underpinings, but even with Arch it could be just a matter of installing Gnome, KDE, cinnamon, mate, following a couple of clearly defined instructions on the Arch wiki for those DEs and I would have a turnkey solution that would (after that) not need to go near a terminal for anything. And thats with a distro that eats noobs for breakfast (supposedly).