He was transported in the One Piece world with a cheat and a Devil Fruit of his design. The cheat he got is designing an OC Modified human of whoever he chose, so he picked a Seraphim, designed merging Garp and Blackbeard  into one as a template, he was then born into One Piece, being the younger son of Garp and half brother to Dragon.

I will rewrite it, as I realized that the Gamu Gamu no mi, would be really similar to Luffy's and I don't want that, so I'll make a different Devil Fruit for our MC


Chapter 3 - Let me rewrite it,