Gerald's scar comes from parental abuse. Draper includes this to highlight the theme of distance between parents and children. Parents are shown to be detached from the reality of their children. Draper's portraits of parents range from apathetic, to unable to forge connection, to downright abusive. Gerald represents one aspect of this dynamic in that his father is an abusive alcoholic. Gerald's scar is the result of being thrown against the radiator. While Andy's parents are nowhere near this bad, the reality is that the manifestation of Gerald's relationship with his parents is a physical one, while Andy's scars are emotional.
This lack of connection between children and their parents helps to enhance the gulf between both. This division makes adolescence for the kids in Draper's work difficult. It also makes their ability to wrestle with profound emotional truths equally challenging. The lack of parental guidance, seen in Andy's situation with his parents, or in Gerald's abusive condition make adolescence even more challenging for the kids in Draper's work. The questions that Andy's brother has when he is struggling to make sense of his brother's death is the end result of this dynamic. It is one in which parents and children cannot communicate on issues where the primary need is communication and the ability to forge connection. When Gerald is thrown against a radiator with the scar that results, it indicates a challenging situation in which parents and children lack open and honest communication. The scar that Gerald wears is a result of this dynamic, similar to how Andrew's suicide is another representation of this condition of being in the world.
Gerald is abused by his stepfather and got the scar when his stepfather knocked him against a radiator.
"I don't know. My friend Gerald - his dad beats him - he's got this big scar on his face from when he had to get stitches when his dad knocked against a radiator."