Leaving the marines what can Garp do to serve justice? They’re the only law enforcement faction that exists and so Garp’s role is to set a good example and cultivate new heroes for the future. Dragon can leave and start forming a revolutionary group because he’s only 17 and dedicated his life to building this force which still isn’t strong enough on its own to bring down the celestials. Garp and Sengoku acknowledge that there’s nothing they can do to change the whole govt on their own so they do what they can to try and enact justice but still have to deal with the fact that their faction has corruption in it, a corruption they don’t have the power to stop. They’re two sides of the same coin one tries to use his status to make a difference, the other tries to lead by example, at the end of the day they can only do so much. Say Garp leaves the marines, you either retire and do nothing or you become some kind of vigilante, not many options if you’re passionate about justice and want to make a difference.