Sorry to tell you but your block is most likely toast...
You must have had an oil pump failure if the oil level is normal. Unless you were cornering hard and for a long time, where there wasn't enough oil being supplied to the bearings, either by centrifugal force or oil pump failure.
No oil on the bearings = spun bearing (when the metal bearing surface contacts the crankshaft journal, and is "pulled" around the crank with the rotation). This will wear down the soft crankshaft material and will produce a knock, which is what you are hearing.
You must have the engine removed from the car to access the crankshaft, and put on an engine stand to have the work done.
In other words, either look for another GSR short block, or be prepared to pay out the ass to have it fixed.
Unless of course you have the time, space (garage), tools and experience to do it yourself. Although this is only recommended for someone who is very experienced.