GM has been putting the gray steel rims with a different tire size and tread pattern for many years on all their pickups 2 wheel and 4 wheel drive perhaps as far back as 2000. There may be an exception on the trucks that have the front electric locking diffs. but I am not sure since the fully locked diff would suffer more stress if one tire was slightly smaller than the other. I guess GM is not concerned about the size difference on the rear diffs.
No 5 tire rotations just 4. On my 2016 I noted 5 tires and rims were to be the same on the offer and when I was taking delivery I looked under the bumper and the spare was grey so after some discussion the dealer brought a matching new rim and tire from another take off and replaced it for me.
On my 22 AT4 its grey just like the rest of them.
For the folks with the electric locking front diff; check your owners manual to confirm.