Gabriel Thompkins is the half-brother of Garcia Flynn.
In the original timeline, Gabriel dies on the same date as the moon landing after being stung by a bee, and suffering a severe allergic reaction.
In the altered timeline, Flynn – while talking to Gabriel's mother, Maria Thompkins, at the park – gets Gabriel an ice cream cone. As Flynn and Maria talk, Flynn mentions how his family was murdered and tells her the killers were never caught. Maria explains to Flynn that if anyone did anything to Gabriel, she'd do anything to find out who did it.
Later, while Wyatt Logan falsely explains to Maria that Flynn is a communist spy, Flynn injects Gabriel with epinephrine to stop the bee sting from killing Gabriel. Maria runs up to Flynn as he does this, punching and pulling at him to stop. Wyatt points a gun at Flynn. Flynn explains that he saved Gabriel from a bee sting. Flynn explains to Maria that he always remembers her as being sad, and after losing a child, he didn't want Maria to lose hers. Gabriel covers his ears as Flynn fires at Wyatt and flees.