His parents were a naval fighter-gunner crew. Like most fighter pilots they died young when their ship was shot down in an ork freeboota raid. He was raised communally by the assorted crew, was trained up by the deck crew and wandering teachers and preachers until he was old enough to fire a mounted cannon and operate an augur array. He worked as a gunner from a young age and became involved in some of the less than legal elements of the station, acting as pilot and driver for some of the syndicates that controlled the outer-rings of the station. In exchange they used nepotism and influence to get him a promotion and a highly lucrative position with a Naval cruiser.
After a 2 year voyage, the ship was waylaid by Pirates, the main ship fled but was forced to leave its support vessels behind. Each vessel took off in a different direction in hopes of reaching a ship or colony, but at sub-light speeds the odds were stacked against them.
Fortunately after weeks in deep space Gunner was picked up by an Explorator ship. By the time he was rescued his injuries from the battle festered badly. The Tech priests amputated and replaced his limbs with crude but utilitarian replacements, a combi-tool and a port interface. They then fitted him with a series of sophisticated MIU interface ports that had also been recovered from the raider captain and for their care, they pressed him into indentured service as payment.
They needed a high grade pilot and by all indicators it was easier to recycle a former pilot than to create one from scratch.
So he was pressed into service for several months, under inhumane conditions. Then one day the shuttle he was piloting suffered a critical reactor breach. the passenger compartment was sealed and the reactor leak was due to vent into the compartment. So he disengaged himself from the controls, cut open an access panel and crawled through the maintenance ducts to manually jettison the reactor at grave risk to his own life.
When they crash landed, the captain he was transporting awarded him by paying off his indentitude and releasing him from service as well as paying for new replacement limbs. In gratitude and out of a desire to repay that debt he signed on as part of the crew of the Final Offer and quickly was promoted to Master Gunnery Sergeant of the ship.