Gil Cassel originated from Bastion and was a male Human whose family supported the Imperial cause. Jared Cassel was his brother, and like Gil, he enlisted in the Imperial Army.
A Human male hailing from Bastion, Gil Cassel's family held pro-Imperial sentiments. Both he and his brother, Jared Cassel, became members of the Imperial Army serving under Emperor Roan Fel. Following Darth Krayt's, the Dark Lord of the Sith, seizure of the Imperial throne, Gil Cassel remained dedicated to the Imperial faction, while his brother switched allegiance to Fel's Empire-in-exile.
By the year 137 ABY, Cassel had attained the rank of lieutenant and was a member of the 407th Stormtrooper Division, stationed on Yinchorr. Concurrently, his brother served as a captain within the 908th Stormtrooper Division. As a component of the Second Imperial Civil War, the 407th was dispatched to Borosk to neutralize the 908th due to the latter's defection. Despite his brother being on the opposing side, Gil still commanded his soldiers in battle.
Amidst the conflict, as the 407th gained an advantage over the 908th, Cassel and Sergeant Ran Harkas of Joker Squad considered offering the defecting 908th the opportunity to surrender. However, Darth Maleval, the Sith Lord overseeing the 407th on behalf of Emperor Krayt, dismissed the proposal and demanded the complete annihilation of the 908th. Despite dissent from Jes Gistang and Hondo Karr, both members of Joker Squad, Lieutenant Cassel decided to adhere to Lord Maleval's directives.
Following the 407th's triumph, Darth Maleval presented the captured Jared Cassel to Gil for execution, intending it as a demonstration of his allegiance to the Sith Empire. However, Gil refused to murder his sibling, leading to both of them being beheaded by the Quarren Sith Lord.