I think Gol D. Roger might have ate the the Gomu Gomu fruit. It make sence knowing how strong Luffy is. And there must be a reason the Shank found the fruit in the first place and KNOWING what it was. And in a SBS Oda said the Luffy ate the SECOND known Gomu Gomu fruit.
I don't know if I'm right or not but you guys give your synopsis on it if you want