Hey DooSPX
I have an RX8 that I added a CarPC to and kept the Factory Bose radio. What you need to do is add an Aux in to your factory headunit. In my case I used a
Car2PC adaptor although there are other methods of adding an aux in to a factory headunit.
The aux in will get PC sound to your factory head unit in my case I change the head unit to EX mode for PC sound. Now as far as keeping your steering wheel controls since sound is being fed to the head unit your volume and mute should still work. The up and down buttons for skipping to next/previous tracks probably won't work depending your your front end. I'm pretty sure if you kind find a forum on the GMC Sierra that should have info about adding an aux in to the factory radio.
P.S
You may not be able to find a Car2PC adaptor for you GMC but I'm sure you can find an aux in.
hope this helps