I accidentally put half a gallon of unleaded gas on a 2015 silverado 2500hd 6.0

Hello, That does not concern me at all. Half a gallon becomes pretty insignificant in a pool of 25 gallons. That's a 1:50 ratio, which is fine, but for good measure, fill up at 3/4 tank two times. That should bring you to a 1:100 ratio, and never another thought should be given to it.