How do you recruit gaius?

To recruit Gaius in the game "Fire Emblem: Awakening," you must first progress through the main story until you reach Chapter 11. During this chapter, ensure that Chrom and Gaius are in proximity to each other during battle, as Gaius will approach Chrom and offer to join your cause. After successfully defeating the enemy forces, Gaius will become available as a playable character. Make sure to engage him in conversation during the battle to solidify his recruitment.