The author wrote this just for fun, so even they don't know where the story is going. Just enjoy it as a bit of entertainment.
He transmigrates to the world of One Piece and becomes Luffy's seven-year-old biological older brother, Lufa!
From the start, he accidentally eats the incredibly rare Human-Human Fruit, Mythical Zoan, Model: Lucifer!
Ding! Fatal damage detected to Host. The Lucifer Fruit has awakened deeply!
Vitality recovery rate +999%! Physical strength +999%!
Divine pressure unleashed. Conqueror's Haki potential evolves: Lucifer's Gaze!
Garp crushes his breastbone with a kick using just a third of his strength? It completely reconstructs and evolves in seconds!
Hunted by the forest's tyrant, a giant tiger, he reveals six wings of light, and the beast immediately drops to its knees in submission!
"Old man, that little bit of strength isn't nearly enough to tickle me!"
The closer he gets to death, the stronger he becomes; the more he gets beaten, the more monstrous he grows. He's the ultimate all-around fighter with no weaknesses!
Ding! After consuming a huge amount of meat, energy conversion rate has skyrocketed, increasing strength a hundredfold!
Garp is stunned, Shanks is speechless, and Ace is devastated when he sees nothing left but bones on the ground!
During the Paramount War, when those wings of darkness spread across the sky, the entire World Government will tremble!
Logically speaking. Cap wanted his grandson to become a Marine. You must be very happy when you say you want to become a Marine Garp because you can protect them. It's entirely possible to write that the protagonist and Garp say that they become a marine so that they can make a living for themselves when the time comes.