How to Acquire Gilbert in Ogre Battle

Go to the third level, which is marked as the "Sharom District" on the world map. Battle with the enemy forces and liberate cities using whatever strategy you prefer, but do not attack the enemy base yet.

Send an army to the town called "Latingur" and listen to a citizen there tell a tale about an Eagle Man named Canopus who was once an ally of the ruler Gilbert. Have another army travel to the town called "Bah'Wahl" to speak with Canopus, who will ask your army a question. Answer however you see fit, as he will not accept your answer no matter what you choose.

Have an army go back to Latingur and again speak with the people there. A person from the town will state that Canopus has a sister who is hiding out in a secret location. Send an army to find the hidden Roshifallion temple at the southwest end of the area, just a little to the left of the enemy base. Canopus' sister Yulia will ask you to convince Canopus to join the fight against the Zetegenian Empire. Say that you will help, and she will give your army an item called the "Wings of Victory."

Once Canopus has joined your armies, battle with Gilbert at the enemy base. After you defeat Gilbert in combat, he will ask to be forgiven for his actions. Choose the dialog option to forgive him, and he will join the rebellion as a playable character.