Gabriel was born on May 14, 1994 to Vivian Maddox. His father, Sam Keating was absent from Gabriel's upbringing due to Vivian and Sam's divorce. When Gabriel was young, his father came round to his apartment looking to be a part of his life. Gabriel's grandmother (Gwen), however, turned him away, refusing to let him know his son. On his way out, Gabriel bumped into his father. Gwen then quickly moved Gabriel out of sight from Sam and into the apartment. When growing up, his mother had mental issues which resulted in Gabriel taking care of her. It got to a point where one of his mother’s doctors was overprescribing pills which resulted in him beating the doctor up. Gabriel was subsequently arrested for assault. Sometime in his later life, Gabriel discovered that he was the son of Sam Keating who had 'died' and moved to Philadelphia where he could be close to his second wife, Annalise Keating.
Prior to attending the University of Virginia where he studied law for his first year, Gabriel attended a separate university for 4 years where he attained his undergraduate degree. Upon discovering his birth certificate when his gran, Gwen died, Gabriel dropped out of the university with the desire to move closer to Philadelphia where he could find out more about his father, Sam Keating's death from his widow, Annalise Keating.
Before starting his second year of law school, Gabriel visits Middleton University for a tour along with other future students. At the sign in desk, outside the university, Gabriel flirts with the signing-in lady. He initially asks for her name and number but the lady refuses to give up anything and asks for his name. After he gives it to her, he shakes her hand and she smiles. Frank in the background, who is also there for the tour, calls someone and tells them that "Her kid is here."
Upon arriving at Middleton University for what is, presumably, his first day, he enters a third-year class and asks one of the other students whether the class is “advanced trial skills.” She mocks him by telling him that if he has to ask about what class this is them he shouldn’t be in it. Annalise Keating then walks through the door and introduces herself to the students nah ma'am don’t already know her past at the university. She introduces the students to a Legal Clinic which she claims will be harder than anything else in their lives, however, only 24 students in the class will be chosen. Despite this, when Annalise asks the class who would like to take part in the clinic, the entire class raises their hand. As a test to see which students have the knowledge and passion to be in the class, Annalise gives the students a task: to express what they are passionate about. Annalise then chooses the first 23 students, until only one seat remains. For the last seat, she is seemingly stuck between Gabriel and Asher Millstone. During Gabriel’s turn, Annalise acknowledges that Gabriel is a 2nd-year student in a 3rd-year course, to which she undermines his capabilities on handling the class. Gabriel tells her that he is only in his second year of law school to try and encourage her to choose him because of his courage to sign up for a third year class in his second year. In the end, Gabriel gets told to take a seat.
Though it was initially believed that Gabriel is Bonnie Winterbottom's supposed deceased son, on a later document it suggests that Bonnie's son is actually white. Gabriel, being of mixed race, is therefore ruled out as Bonnie's son.