The gas block set screw looks A LOT loose.I noticed my G3 gas valve was no longer flush after a day on the range, did not have any cycle issues I may add. the detent was protruded as well. I emailed in, and was told the gas valve heat treat process had changed and I would need to send the barrel/block in RMA to be repaired. Upon closer look the valve itself was cracked and the set screw could not be removed. The set screw on the bottom of the gas block was also a little lose. I bought this weapon new and had about 5-600rds mostly suppressed in 2 range trips. 12in barrel.
Well, as I said, it gets worse...before it could reach my destination, the barrel was removed from the package I sent it in and no where to be found, and now I am waiting on another barrel to be made...WEAK
Did the dial itself crack or when you say 'valve' do you mean the internal part of the dial that has the different gas port sizes?
I'm assuming since you said the set screw couldn't be removed that you didn't drill it out and disassemble far enough to find out if the internal stem/port assembly was damaged. I do wonder if the loose gas block set screw could contribute to your breakage....as cycling the weapon would have the piston driving into internal parts....and trying to push the dial off the stem, resulting in what looks like (the picture isn't clear) a crack in the dial at the set screw location.
Did you ever check to make sure the gas block set screw was properly torqued? They usually don't loosen up so I'm guessing as new as the rifle is, someone at the factory forgot to tighten it appropriately.