The theme of this story is, quite clearly, love and human decency. It is a story (like many of O. Henry's) about what people do (and should do) for love.
Young Gillian is characterized as something of a playboy. He clearly wastes money at clubs and on showgirls. His uncle has not liked this about him and has left him $1,000. Gillian doesn't know this, but if he spends it well, he gets $50,000 and if he doesn't he gets nothing more.
Gillian spends the money well -- gives it to the woman he loves (even though she doesn't love him). Then he finds out about the $50,000. He finds out that if he spent the money badly, the $50,000 goes to the woman he loves. When he hears this, he lies and says he wasted the money.
Gillian shows the best side of love here. He knows that Miriam will not love him no matter how rich he is. So he sacrifices his happiness for hers.