GGG would have been Pirog’s toughest test, outside of Jacobs, Pirog’s opposition wasnt that great except maybe Jantuah, though they were ok Pirog rarely lost rounds and adjusted, the GGG of 2013 had a granite chin and the one punch KO power, hard to tell if Pirog would have made the necessary adjustments, (it was a difficult thing to do in 2013 against GGG), and I think it was going to be Pirog’s first fight at 160 so there that also.
As others here mentioned, Pirog was the better boxer and quite crafty for a Russian fighter but GGG at that time was steamrolling guys who were a step below Pirog, I think GGG would have settled for a UD after putting Pirog on his ass a couple times.