Gabriel was born in Fort Rucker, Alabama. As a child, he had to travel around the world because of his father Albert's service in the U.S. Army. He also had a mother who died of breast cancer and a sister named Tara. As a child, he and his family moved to Germany. There, Tara was harassed by a bully named Martin Gumbel, who made fun of her name and drew a swastika on her locker. When Gabriel reported it to the school administration, Gumbel and his friends sought revenge by beating him up. As a result of the incident, Lewis admired her brother for always standing up for her, while he started calling her "T" to show her he would always support her, regardless of the pronunciation of her name. Tara would later recount elements of the story while interrogating Antonia Slade (Devil's Backbone).
As an adult, he had high intelligence alongside Tara and was given a scholarship to Yale University, though Gabriel chose to turn it down. Though Tara believed he turned down the scholarship because he wanted to get rich quickly, it was also because Gabriel was traumatized and then jaded by an incident where he had to stop Albert, drunk and despondent over his wife's death at the time, from killing himself with his shotgun. Later in life, Lewis had to help Gabriel through the aftermath of one of his get-rich-quick schemes and tried to get him to change his ways, but her efforts were unsuccessful, leading to an estrangement between the two. In 2016, he partnered up with who he believed to be a man named Carl Brubaker, in an effort to lobby for the legalization of online gambling. Unbeknownst to him, Carl was actually Peter Lewis, a serial killer by proxy who escaped from prison. Peter abducted Gabriel and brainwashed another man, Desmond Holt, into assuming his identity.
The BAU launch a search for Gabriel after finding out that Desmond is posing as him. Tracking down an address rented by Peter under an alias, Lewis and Alvez find a TV screen showing Gabriel being held captive by Peter. By the time the BAU find Gabriel at a warehouse, Peter has escaped after setting up a booby trap with a device that fires nails from the ceiling. Despite this, they are able to rescue Gabriel. Afterwards, Gabriel is brought to the BAU headquarters, where he is reunited with Albert.