The relationships of Gil throughout the Descendants franchise.
Gaston is Gil's father. Gil seems to admire his father a lot.
The Gaston twins are Gil's older brother. It's unknown the relationship between these siblings.
Harry and Gil used to be friends with Uma when they were children. The boys usually follow Uma's orders, eventually becoming a habit. As they grew up, Gil became distant and stopped hanging out with them. The three friends reunite once again after Uma won the Lost Revenge and became captain of her own pirate crew.
In the junior novelization of Descendants 2, Harry and Gil kissed. This scene was cut from the final version of the film. This could have indicated a romance between them.