How weak is Garp if pre-timeskip Luffy made him bleed? Not weak at all.
It’s just that people become vulnerable to taking more physical damage when they become emotionally vulnerable/overwhelmed/damaged. This is especially common with the top-tiers. Examples follow.
1- Garp in Marineford
He was emotionally burdened when he reflected on Luffy and Ace’s childhood so he took more pain from Luffy’s punch when otherwise Luffy’s punch wouldn’t even tickle him.
2- Big Mom
When Mother Carmel’s photo was broken Big Mom was emotionally overwhelmed and the mere weight of her body was enough to make her knees bleed, when even powerful canons/bullets/swords would have no effect on her otherwise.
3- Kozuki Oden
When Oden was about to deliver another blow to Kaido, Momonosuke’s cry for help caught him off-guard and there was a visible look of apprehension on his face so Kaido’s one blow took Oden out.
People like to assume that super powerful people are super powerful 100% of the time, this is simply not true. One Piece is more realistic than people give it credit for in this regard because even the strongest in the series are never that absurdly powerful.
There isn’t a single human being in the world capable of being in peak condition 100% of the time, and when a human is vulnerable they can be destroyed with relative ease.
*Theory by AllBlueReverie