This is a very interesting question! As we read the story, it is evident that Gillian is in love with Miss Miriam Hayden. Miriam is the former ward of Gillian's deceased uncle.
When Gillian is willed a thousand dollars by his uncle, he appears at a loss as to how to dispose of the money. However, when he discovers that Miriam has only been left a seal ring and ten dollars, he decides to bequeath the thousand dollars to her as a gift. After he hands the money over to the surprised Miriam, Gillian proceeds to provide a written record of his expenditure to the lawyers for their perusal. However, when he realizes that, upon prudent expenditure of the thousand dollars, he will be further bequeathed fifty thousand dollars, Gillian proceeds to tear up the written record of his expenditure.
Although he appears nonchalant and careless, Gillian privately harbors more substantive feelings: he is in love with Miriam. This is why he decides to give her the thousand dollars and also why he neglects to inform the lawyers about his actions. If Gillian had confessed his unselfish action to the lawyers, they would likely have released the fifty thousand dollars into his care. Instead, Gillian's act of love ensures that his beloved is amply provided for in the event of his uncle's death.