Born in Stillwater, Pennsylvania, Gibbs had a quiet childhood until tragedy struck his mother. Ann Gibbs (Clare Carey) committed suicide by intentionally overdosing on drugs. While she had done so to spare Gibbs and his father Jackson (Ralph Waite) from watching her die of terminal cancer, the act would leave Gibbs devastated, and it destroyed the relationship between father and son. It would be decades before the two men spoke regularly. Shortly after graduating high school Gibbs would enlist with the Marines. Less than a year after his enlistment, he lost a close friend in a helicopter crash near the naval base at Okinawa.
These wounds would heal as Gibbs became a military police officer and began dating a young woman named Shannon Fielding (Darby Stanchfield). The two met in Stillwater, and Gibbs had stayed in touch, and the pair fell in love and eventually got married. Two years later, they had a daughter, Kelly Gibbs (Mary Mouser).
But the happiness didn't last. While Gibbs was away fighting in Operation Desert Storm, Shannon witnessed the murder of a fellow Marine by drug cartel leader Pedro Hernandez (Thomas Rosales Jr.). Despite the danger to herself and Kelly, Shannon chose to testify in the case against Hernandez. As the pair were being taken to a safe house by NCIS agents, their car was attacked and crashed, which killed both of them.
Source : https://www.looper.com/644237/how-a-series-of-early-tragedies-shaped-gibbs-character-on-ncis/241