Amy: You managed to free them. Thank you. We are friends from now on. Now you and your brother can live together again.
The prison cutscene. The supposedly intimidating robot confronts Amy for the bird, after she refuses, and in desperation, actually begs him to let it go. Gamma starts feeling pain from his new found empathy, going berserk before letting them out of their cell and ordering them to escape. He slumps over the door switch, dumbfounded and guilt ridden at disobeying his master's orders, though Amy shows sympathy and offers him her friendship before leaving. Gamma's heartwrenching theme playing in the background helps none.
Keep in mind, the E-100 units in Sonic Adventure were essentially siblings. Gamma was forced to kill the only people he ever cared for — with the exception of Amy and his creator Eggman (who he probably saw has his father) — for the greater good of saving the birdies from a Fate Worse Than Death.