Aftermarket. It's definitely not as sturdy as the OEM that was on the truck, but I don't see that being an issue. The finish looks really good.
This is the one I got http://www.ebay.com/itm/Denali-Style-2014-2015-GMC-Sierra-Sierra-Denali-1500-Chrome-Mesh-Grill-Grill-/331748401389?fits=Make%3AGMC%7CModel%3ASierra+1500%7CSubmodel%3ADenali.