This outburst is the most emotion we see out of Garp this war. Since visiting Ace in Impel Down we've seen little of his boisterous nature, and I think in addition to being sad, he was emotionally preparing himself for Ace to be executed. A flip was switched when he threw the fight with Luffy where he for the first time chose his family over the marines, and like the rest of us I think he expected Ace to be home free after being released from his cuffs.
I think at the end of the day Garp is inherently selfish in the way Luffy's selfish. He wants to go out and do as he pleases--which in his case is be a marine and hunt down pirates--and refuses to make it any more complicated than that. But unlike Luffy his selfish desires clash with the fate of the people he loves. While Luffy brings his found family under his wing as he goes out on grand adventures, Garp's adventures never include his loved ones in a positive way. Garp wants to both care for the people he loves and follow his heart's desires, but he can't do both, and it's one of the reasons his grandson ends up dead









