Quote Originally Posted by Kato View Post

Also, I totally had the same theory as VanBuren. On a slightly more serious note: How can he be not Luffy's dad? Garb definitely got something mixed up there...
Yeah, but Luffy got the hat from Shanks, who served with Roger if memory serves. Roger told Garp that was the woman carrying his son, and presumably Dragon turned Luffy over to him, which is why he knows that Luffy is his grandson. Add in that Luffy is younger than Ace, that means in order for him to be Roger's son his mother must have been pregnant with him for 2-3 years. So either Garp has been lying to everyone this entire time and Shanks is the only to find out, or Shanks just decided that Luffy reminded him of Roger, and so gave him Roger's hat.