Shiryu would never lower himself to some 1v1 like a prideful moron. If Garp tried to fight Shiryu, he would just vanish, wait until Garp was distracted and then slice his head off or some shit lol.
Fair 1v1s are for idiots and Shiryu is above that. Given that Garp bleed from Luffy’s weak ass, I’m pretty sure Shiryu could kill him instantly with a well-timed sneak attack.