Gabe is Percy's stepdad, and, as his last name indicates, he is ugly in body, mind, and soul. Gabe hates Percy's guts. All he likes to do is sit on his booty all day, eating chips and dip, and playing poker with his buddies. He takes money from Percy and from Percy's mom, Sally. We learn that the primary reason why Sally married Gabe was for his stench. His smell is so strong and overpowering that it helps mask Percy's demi-god smell from monsters.
When his camaro is totaled the night Percy enters Camp Half-Blood, and when both Percy and his mom go missing, Gabe helps paint a portrait of Percy as a "troubled" kid with lots of "issues," and probably responsible both for the car wreck and his mom's disappearance. When Percy finally returns home after completing his quest, Percy realizes that Gabe hits Sally Jackson on occasion. But he gets what's coming to him. After Percy has returned to Camp Half-Blood for the summer, his mother writes him a letter telling him that she used Medusa's severed head to turn Gabe into a statue.