When Gray stabs Jack, he brings the weapon up from below Jack's rib cage. When Jack is seen lying on the ground, the stab wound is from straight on in Jack's chest. It should have been a slicing wound, starting from below the rib cage.
Jack Harkness is not Captain Jack's real name, as he's from the Boshane Peninsula in the 54th century; his real name has yet to be revealed. Yet when his brother Gray finds him and Captain John Hart (in the year 27 AD in what will one day be Cardiff), Gray calls out "JACK!" As his brother, Grey would certainly know his real name and would not call him Jack.