One Piece's title refers to a treasure left behind by Gol D. Roger, the late pirate king. He is frequently described as the strongest pirate to ever exist, and his execution only possible due to his consent. Only a few characters in the series are confirmed to have traded blows with Roger and walked away alive. Chief among them are the deceased Yonko Whitebeard and Luffy's grandfather, Vice Admiral Monkey D. Garp.
Garp has a long history with both Whitebeard and Roger. He and Roger teamed up against the Rocks Pirates (of which Whitebeard, Big Mom, and Kaido were all members) in a battle known as the God Valley Incident, which led to the dissolution of that crew. The Rocks Pirates were targeting the World Nobles, and thus Garp's actions in the battle earned him the title of «Hero of the Marines». Garp would fight and attempt to capture Roger numerous times after that. Prior to Roger's execution, Garp agreed to care for his son, Portgas D. Ace, raising him along with his grandson Luffy.
RELATED:One Piece: How Strong is Rob Lucci?
The mere fact that Garp traded blows with some of the most notorious pirates ever and lived to tell about it speaks to his tremendous strength. However, most of this history is spoken about but not directly shown throughout One Piece, and thus the exact nature of Garp's abilities are poorly explained. All of Garp's exploits so far involve raw physical power and durability. He took no serious damage from a punch thrown by Luffy in Gear 2, despite it sending him flying, and barely noticed when the former Marine captain Axe-Hand Morgan slashed him.
Garp has been seen throwing cannonballs with his bare hands at speeds that surpass those that they reach when fired, and effortlessly swung a giant iron ball
Read more on gamerant.com