Garp is presented as a good guy because he:

1. Refuses to be an admiral, so he wouldn’t be a pawn for the Gorosei.
2. Refuses to be present for the CDs, whether in general or on genocide games.
3. Is heavily invested in training young bloods who will change the Marines from within.

That said, even though I don’t think he’s a bad man at all, I think he’s a gigantic idiot. He knows about the genocide games, he knows his closest friends okayed heinous crimes (Ohara buster call), he witnessed fellow Marines going around killing infants and pregnant women around the world as he personally protected Roger’s true wife. All of this should completely break him and think that his son Dragon is doing the right thing, and that even his grandson Luffy is being a better influence on the world than the WG have been in recent history.

I personally detest Koby and think he’s irredeemable.