You can use a PS3 or XBox 360 controller without root, BUT you do need to buy an USB OTG cable (Amazon.com: micro-USB to USB OTG Adapter Cable for Samsung Galaxy S II (AT&T, T-Mobile & i9100): Cell Phones & Accessories) either controller has to be used WIRED and plug into the USB OTG cable. The charging cable for the PS3 works and a wired Xbox controller would work also a wireless Xbox 360 controller works, but only if you have the PC receiver (Amazon.com: Xbox 360 Wireless Gaming Receiver for Windows: Video Games).
If you are rooted there is a paid app (sixaxis) that will allow you to use a PS3 controller wireless (after you pair it to your N7).
Hope this helps.