For most of the story, Gillian is shown as something of a playboy. He is presented as a person who is only interested in having fun.
When he is given the money right at first, he seems really spoiled -- he's complaining that it is an awkward amount and he'd rather have some different amount.
At the club, we see that people think he's something of a pain. Then we find out that he is considering spending all the money buying a gift for a show girl.
Finally, we see that he is so much like this that his uncle is testing him through his will. His uncle wants to punish him if he keeps acting like this.
But when Gillian finds this out, we see the other side of his character. This is when he does the right thing and lies so that Miriam will get all the money