The majority of the upper rapids are fairly easy; one move and you are through. The more difficult rapids start at the Boulder Gardens and continue through the Narrows. I have run it a few times, and would compare the lower stretch to Shoshone in difficulty. However, I have never been there when the flows are greater than 1500, so depending on when you go will determine how useful this info is to you.
Using Imaciag's info will be of great use to you. The first time I ran Gunny Gorge, I actually carried the BLM descriptions of the rapids with me. They add a little hype to the intensity of the rapids (in my opinion), but considering the difficulty of a rescue, I can see why the would do so.
The hike isn't terrible - probably a little over a mile. Last time I checked, the shuttle was around $75/vehicle. If you are going to arrange a shuttle, make sure to do so in advance and check in before you head down to Chukar. We tried to get in touch with Leroy at Pleasure Park to run shuttle after half of our party bailed, and the number had been disconnected.
Hope this helps....