Jack Brennan served in the United States Marine Corps and later joined the Federal Bureau of Investigation (FBI). During his time with the FBI, he worked as a sniper and also served in the agency's elite Hostage Rescue Team (HRT). He retired from the FBI after a distinguished career and subsequently worked as a private investigator and security consultant.
The creator of "NCIS," Donald P. Bellisario, was inspired by Jack Brennan's life story and experiences when developing the character of Leroy Jethro Gibbs. While Gibbs shares some of Brennan's traits and characteristics, it's important to note that the character is fictional and not a direct portrayal of Brennan himself. The writers of the show created a unique character that incorporated elements from Brennan's background while also adding fictional aspects and storylines.
However, Jack Brennan's contributions and experiences in law enforcement and military service served as a source of inspiration for the character of Leroy Jethro Gibbs, helping to create a believable and compelling protagonist for the "NCIS" series.