To the best of my knowledge we aren't offered any back story to this character. He appears in Episode 01 Guadalcanal as an ancillary character, and it's not until Episode 04, Pavuvu & Banika that he has any kind of a significant role.In Episode 04, while the 1st Marine Division is fighting in New Britain, Gibson is part of a squad checking out an abandoned Japanese medical aid station Gibson chokes to death an injured Japanese soldier. We next see him in segregation on Banika, when Leckie goes to Banika for treatment of his enuresis. We're left with the impression that Gibson had a nervous breakdown while on Pavuvu and will never return to battle.Presumably the writers intended for Gibson to be an example of those Marines who cracked wide open and couldn't return to battle, and Leckie is an example of those Marines who only had a hairline fracture to their shell - and were able to get their bearings back and return to their unit and the front lines. Watching Gibson's reaction, his sense of shame is clear.