When playing mission 2 in story mode, shoot down at least five of the Dopps that try to shoot down White Base. To make this easier, jump up on to White Base and shoot down the Dopps as they fly toward you. Once you have accomplished this, and the level progresses, instead of flying in by a red Dopp, Garma will be in his Zaku. Defeat him and successfully complete the rest of the mission to unlock this mobile suit in the gallery. If you have all the Zeon suits unlocked in the tactics battle mode already, then just finish a level to unlock this suit in the tactics battle mode. If not, just progress through tactics battle mode as usual.