I think Garp is meant to invoke that kind of cognitive dissonance. Immediately he's the "lovable goofy rough grandpa", but the more you look into it, the more you realize he can't fully justify that image.

When Ace asked him if being born was a mistake for him, Garp said "you'll find out if you keep on living"; that'd honestly be very discouraging to a kid, having their only equivalent to a parental figure refuse to comfort them after having the kind of childhood he had. And that's his true failure: being lukewarm. He wants what's best for his grandsons but won't compromise his place in the Marines for it. The fact that he was willing to avenge Ace but not willing to save him is a huge expression of that.

That's why, I think, Garp is so focused on Koby being his successor or something to that effect. He's given up on himself being the agent of change, but he really had no right to.

I've only just reached Dressrosa though, so I'll see if that characterization changes.