Monkey D. Grap went to Hachinosu Island to Save Kobi from Blackbeard Pirates. During their escape, Garp, while trying to save Kobi, was first stabbed in the gut by Shiryu, which resulted in a fatal wound. Next, we went head to head with his student Aokiji (or Kuzan), and it is currently unknown whether that resulted in Garp's death in chapter 1088. So, we have to wait to learn about Garp's killer in One Piece.