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In 2553, Gallo and the rest of the 717th Xeno-Materials Exploitation Battalion was posted to the Outer Colony of Gao to retrieve Forerunner ancilla Intrepid Eye on the planet. Gallo, along with fellow Sierra Company infantryman Lance Corporal Ryan, were frequently assigned to guard Blue Team's Portable Spartan Support Module near the United Nations Space Command's facilities at Montero Vitality Center.
On July 6, 2553, after Intrepid Eye was captured and stored in the PSSM, the ancilla took control of the module after destroying the battalion's smart AI, Wendell. Attacking the humans inside the module, the ancilla killed most of the humans inside the PSSM and trapped Special Inspector Veta Lopis in a depressurizing airlock. With Gallo and Ryan attempted to open it, Frederic-104, Ash-G099, and Olivia-G291 arrived at the module and broke into it, freeing Lopis. However, Intrepid Eye operating an inspection drone and attacked Fred. As Ryan attempted to help, the ancilla slashed his throat with its utility tentacles. As the Spartans attempted to subdue the drone, Gallo fired at it with a battle rifle, furious at the wounds the ancilla had inflicted upon her friend. Fearing that she might attempt to further damage the ancilla, Fred had Gallo fruitlessly help Huragok Roams Alone attend to Ryan's wounds.
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