Make sure you also check out Gnome Shell's killer feature : extensions.gnome.org

I wish there were an amazing guide for building these shell extensions somewhere. I tried to look for some and couldn't find any that really got into the details.

sudo apt-get install ubuntu-gnome-desktop

be aware that this isn't a great user experience - a number of ubuntu specific features will work poorly or not at all while you're logged in under gnome shell, and several gnome 3 desktop integration features also won't be working. It's mostly fixable, but very much a trial and error experience.

Can installing Gnome Desktop break something in Unity?

I.... don't think so. It's hard for me to remember for sure, but if it did it wasn't anything big.