Gruff, spunky, and hard-headed even as a child, Gilly did not want to attend OCS but was not given a choice in the matter by her parents, both of whom were reputed veteran officers. She doubly resented this expectation due to her older sister having already begun her own career as a junior officer by the time Gilly was sent to Corbulo Academy in 2492. Following the deaths of her parents that same year, she endeavoured to honour their wishes but ultimately dropped out in the second year of her Accelerated Education studies, choosing to enlist into Naval Special Forces instead. Regardless, her extraordinary record in field training was noted by her teachers, and Project: ORION maintained a steadfast interest in her as a potential candidate when she came of age. She became the second-highest ranking candidate within the program by 2495, excelling in every form of combat but particularly with hand-to-hand and CQB.
Gilly remained in military service into the early 2500s, performing at peak functionality even as her fellow ORIONs began to exhibit undesirable side effects from their augmentations. However, she was pulled from active duty in 2521 after utilizing excessive force on Insurrectionists in public, and placed under the watchful supervision of James Lee. The two began a romantic relationship, after which point he deduced that Gilly was suffering from neurological discrepancies that were causing her bouts of psychosis, but they kept the matter quiet as she generally functioned lucidly in his presence. Gilly was cleared to return to full service when war broke out against the Covenant, but this only accelerated her mental decline until she received an other than honourable discharge in 2532. When she gave birth to her daughter Janissary in three years later, she decided to keep the news a secret from ONI, prompting Lee to retire and go into hiding with Janissary under a different name. In 2538, Gilly experienced a psychotic break and was deemed to be unfit for civilian life, losing custody of her daughter and being committed to a psych facility by ONI, where she was placed under constant medication after her fraying psyche was deemed unsuitable for cryostasis. She had no ill will about the arrangement, recognizing the danger she posed to the public but still willing to take the fight to the Covenant if humanity needed her again.