Gabe is known for being a troublemaker, sarcastic at most times, and apparently lazy. But he is also known as cunning and determined.
Despite his usual troublemaking and pranking, it is shown in "Sun Show, Part One and Part Two" that Gabe is capable of doing good things. This is shown when he promises in the helicopter crash that, if God lets him live, he would do something good. Immediately, the helicopter pilot, after being in a narcoleptic episode, wakes up and takes control of the plane. During the rest of the episode, he keeps his promise to God by helping PJ overcome his fears. In "Special Delivery", he refuses to steal candy and the Kuddle Me Katie doll Charlie wanted, despite Jake telling him to.
Gabe also has a sort of Machiavellian side to his personality, known to switch sides, and joining anyone if he felt it could benefit him, for example, one occasion he gave his mother, the information she required to spy on PJ, then sold him the knowledge that she was spying on him, in another he told her and Mary Lou Wentz how the girls tricked them, and encouraged them to go after, only then to warn the girls, so both of them would owe him a favor.
He is described as sharp as a tack by his dad.
Gabe is also a member of a basketball team that Bob coaches, and even through his negative reputation at school Gabe was voted class president ("Teddy's Little Helper").
While Gabe was in no way excited for Charlie's arrival and disliked her even well into the first year of her life ("Study Date"), he grew to love her and even involve her in his zany schemes.
Gabe often makes fun of his parents' age and weight. Once when he was not allowed to make fun of Bob's weight for a week, Bob concluded that that's never going to work. HE wrote many fat jokes about Bob in a week, which Amy liked. He still loves him anyway ("Gabe's 12-½ Birthday").
Gabe relies on Amy to do tidiness work for him. He often calls her to get the door, and once, implies that she makes his bed every morning ("The Curious Case of Mr. Dabney", "Duncan vs. Duncan"). He told her that he must get his evil genes from somewhere when he helps her spy on PJ's buddy page. He loves her and seems to be more of a mom's boy.
Gabe does somewhat love PJ but often manipulates him to help him get money and do work for him. He is very close to him, although he makes fun of him a lot for not being "very thinky" (because Amy won't let him call PJ "stupid").
Gabe thought Teddy is a loser, initially because she is a middle child ("Study Date"). However, later on, their relationship can be described as neutral.
It is known that he takes pictures of Teddy's activities with her boyfriends to embarrass her in front of her schoolmates ("Meet the Parents"). He helped Ivy Wentz come up with the nickname GG (goody goody) because Teddy well behaves at school.
Gabe has a complicated relationship with Jo. They had crushes on each other when they first met, her treating him badly because of anger issues and frustration. After he cheated at a video game, they stopped their relationship. ("Boys Meet Girls") He has had certain dreams about her, possibly about maintaining friendships or even a loving relationship with her. He views her as a bully. He let her film a campaign video which turned out to be a campaign video for her own campaign on their class page ("Teddy's Little Helper").
At a cotillion, Gabe launched melon balls at Jo with various spoons and eventually hit Mrs. Krump. She made them the first ones to demonstrate the dance positions. He thought it was gross to dance with her and said he didn't want to be her boyfriend. When she returned his corsage, he came back with it and danced with her. When she put earthworms in Mrs. Krump's salad, he said she really is a delight ("Duncan vs. Duncan").
It seems that Gabe will have a better relationship with Toby than he did with Charlie when she was a baby. It was seen that before he was born, he said he wanted a little brother to pick on. When it became known that he was indeed a boy, he was excited, and his mood was even better when it was revealed that his name was Toby, the name he suggested for him in "Name That Baby". He suggested that name because it's "cool."
- His age has been a discrepancy in the show's continuity. While it is known that all characters develop and age in the same time as the real world, Gabe has been referred as 10 and in third grade at the beginning of Season 1 ("The Curious Case of Mr. Dabney", "Dance Off") and 11 in November ("Teddy Rebounds"), Gabe is in fifth grade ("Teddy's Bear") and even an entire episode on his 12-½ birthday ("Gabe's 12-½ Birthday"). It is mentioned he is 13 in Season 3 ("Name That Baby").
- He has an e-mail account ("Charlie is 2!").
- His online account on the video hosting site is called "Gabe D." ("Charlie Goes Viral").
- He has a history of watching scary movies and blaming his mom for it ("Sleepless in Denver").
- He's the only Duncan with brown hair as the rest of his family's blond/blonde, though Toby might have brown hair.
- He has had two friends on screen who have appeared at least once who are actors on the ABC sitcom Modern Family: Rico Rodriguez, and Nolan Gould.
- He seems to be very good at manipulating people as seen in various episodes, like ("Appy Days").
- Beginning in season three, his voice got deeper.
- His middle name is a joke on the Star Wars character Yoda.