Jono explained his new idea in a blog post, "You want the very best computing experience, so you first go out and buy a Mac. They have arguably the nicest overall hardware combo (looks, usability, battery etc) out there. You then download a distribution from the Internet. This is shipped as a .dmg and you install it. It then proceeds to install a bunch of software on your computer. This includes things such as: GNOME Shell, All the GNOME 3 apps, Various command line tools commonly used on Linux, An ability to install Linux packages (e.g. Debian packages, RPMs, snaps) natively." Basically, GNU/OSX.
Jono says he envisions it as looking and feeling like a Linux box, but atop Mac OS X. In that regard, you can skip out on Linux hardware drivers and utilize what's shipped by OS X. He thinks this project could be beneficial in exposing more free software to the OS X user-base, could be good for smaller Linux desktop enviornments, and bring in a lot of new development attention to projects.
He's just tossing out this idea via this blog post to encourage discussion and see what ends up happening. There are some similar projects like Fink, Mac Ports opens up a lot of free software to OS X users, etc. Nevertheless, it would be fun to see GNOME Shell running on OS X.