I have a 2008 GMC Sierra that shows a code P0306 misfire on cylinder 6

A mis fire is a misfire, which means the ignition coil could be bad, and if you check your service records, compare them to your vehicle's mileage. When doing this, remember that most cars can get 30 to 45k miles before changing spark plugs. Have a mechanic or if you're a DIY person/fan, and have tools, check sparkplugs and sparkplug cables with a wire tester. If all wires are good, then the light will come on because there is current running through the wires. If any of them don't light up the wire tester, that wiring needs changing. Lastly, sparkplugs come in a box and let you know about how long they'll last, depending on driving/road conditions. Your vehicle may be overdue for a tune- up.

I think you have to set your idle a bit higher than what it is now