For Harry, you could explore his family background more deeply. Maybe he discovers some long - lost relative on the Isle who has a different view on life, which makes Harry question his own actions and values. For Gil, he could be developed by having him learn a new skill, like fishing in a more sophisticated way, which could also give him more confidence.
In fanfiction, Harry and Gil could be put in situations where they have to make moral choices. For instance, they come across a person in trouble and they have to decide whether to help or not. This would add more depth to their characters as we see how they handle ethical dilemmas. Also, their relationships with other characters could be explored further. If Harry falls in love with someone unexpected, how would that change his relationship with Gil? And vice versa for Gil.