Its obvious why Luffy and Dadan were able to make Garp bleed, cuz Garp didn't use any haki against them, just like how sanji cant use his haki against female characters, and Morgan could do it cuz Garp was sleeping, and if anything it showed how even if Garp is actually hurt, he is so durable he didn't even wake up, his actual duarbility is crazy good when he is also using Haki to defend himself