Monkey D. Garp and Sengoku are legendary marines, who were able to fight on par with the Pirate King, Gol D. Roger and his crew (including Dark King, Silvers Rayleigh).
Seeing the fear and admiration that Rayleigh drew at Sabaody Archipelago, one can surmise that Roger was even more powerful. Garp and Sengoku were the only ones powerful enough to go head-to-head with Roger Pirates. They also stopped Shiki at Marineford while everybody else panicked. Garp defeated and nearly killed off Don Chinjao, whose bounty was worth around 500,000,000 Beli in his prime and was said to be able to split a continent with a headbutt.
He claimed to have crushed eight mountains during his training to face Chinjao. They held Marco, Burgess and Blackbeard on equal footing. Sengoku actually held Garp down to prevent him from killing Akainu. So it goes to show that Garp can defeat Akainu.
