Garp knows that he is not capable of doing anything on his own, even knowing the truth about Imu and the atrocities committed by the Tenryuubito, going crazy and defeating a couple of Holy Knights will not solve anything, it will only be a drop in the ocean. He probably knows that in the future there will be important pieces (Luffy, Cobby etc) that will be the key to defeating Imu and changing the world. He thought the best way to help was to swallow his pride and stay in the navy to make sure, thanks to his contacts, that those pieces can reach the king (Imu).