Garp is a deeply misunderstood character and definitely one of the most nuanced ones Oda has written in the series. This man has gone through so much. He’s a tragic hero.
First and formost the guy absolutely SUCKS at mentoring the younger generations. Every single one of them deviated from the path he wanted them to follow.
Dragon, his son, is the most wanted man in the world and head of the Revolutionary Army with Sabo as his second in command. Aokiji, his former protege, who left the marines and is the 10th Titanic Captain of the Blackbeard Pirates, one of the most heinous pirate crews.
Luffy, his grandson, became a Yonko, punched a Celestial Dragon, and declared war on the World government. And Ace, his adopted son, who joined the Whitebeard pirates and was executed by Akainu.
I can’t imagine the grief and burden that Garp has to deal with. He’s a man committed to his job and chooses his duty over friends and family because it’s the “right” thing to do. And it kills him inside every time. First, Ace’s death and now he has to face Aokiji, who Garp may kill or who may kill Garp.
I think that’s what makes Koby so special though, he’s the legitimate successor to Garp’s will. Koby is truly Garp’s last hope for the future of the marines. Koby is Garps victory.