OK, I am new to this Phantom 2 Quadcopter. I purchased this from a guy who crashed it and I got it all up and flying. I would like to add a GoPro Hero 3 (which I already have) to the Phantom 2. I know I will need a Gimble to mount the camera.

What else do I need to be able to view on a device?

If by "device" you mean smartphone or tablet, then you cant.
What you want is one of the many FPV solutions on the market that come with a monitor to view  the video.

The phantom 2 radio control is on 2.4 GHz which is the same frequency the cell phone and tablets use for wifi. So to be able to see when you are flying you need to use first person view or FPV which is on 5.8GHz. Now the phantom 2 vision and vision plus radio control is on 5.8GHz so that's how you can use the wifi on 2.4GHz.

Not sure what model GoPro you have but if it's a Hero 4, get the H4-3D gimbal, if not get the H3-3D. For FPV, I use the Immersionrc 600mw 5.8 GHZ transmitter with 7 channel Black Pearl Monitor (not the 32 channel). Spironet antennas on the bird and monitor with an additional patch antenna on the monitor. I have gone out to 1 mile and still get good video. Check out  http://www.dpcav.com/xcart/Black ... om2-iOSD-Mini.html. they have a complete package which is plug and play. Also consider the mini IOSD so you will have telemetry display on your monitor...battery remaining, heading, altitude, distance, orientation.