I am having the same issue with my iPhone 6 (iOS 8.4) on my 2012 GMC Terrain. Through investigation I discovered there were two systems available for this vehicle, the standard Color Touch Infotainment system and the Color Touch Intellilink system found in the higher end 6 cylinder models. The Intellilink system can be updated via USB stick by the user. The Infotainment system cannot. I have the Infotainment system in my 4 cylinder model of the Terrain.
Both my and my wife’s iPhone 6s running iOS 8.4 will not function in the Terrain via USB. They do work using a cable from the headphone jack to the Terrain’s Aux In jack but of course no display, just music. They work fine with the Phone app and Bluetooth pairing, just not the Music app. The same phones work just fine in our other car with an aftermarket Kenwood installation. So it’s logical to assume the problem lies with the GMC Terrain firmware.