Fyodor Dostoevsky's novel The Gambler, as the two earlier contributions note, is about addiction, obsession, money, and loss. Dostoevsky himself was addicted to gambling and wrote this work primarily to pay off gambling debts.
The first striking thematic element is the setting of the novel in Roulettenburg, a German resort, despite the characters being Russian, French, and English. Part of what makes them so susceptible to the pleasures of gambling and other obsessions is their rootlessness, and detachment from any real sense of community or belief system.
The next theme is the role of Russian émigrés and the ways they are looked upon outside Russia. In this period, Russia was considered wild and barbaric by Europeans, and wealthy Russians visiting Europe as exotic and not quite civilized. They were, in many circles, only socially accepted for their money.
Wealthy, educated Russians often had an ambivalent attitude towards their homeland, living abroad, speaking French, and imitating European manners and customs, while neglecting their Russian estates, and exploiting them to finance a luxurious expatriate lifestyle, despite having a fierce pride in the idea of Russia. Because money was so necessary to social position, even people with limited financial means would live extravagantly to gain social acceptance in aristocratic circles. At the opening of the novel the narrator states:
Whenever money was in hand, a banquet in Muscovite style was always given. ... The party had already gained some notoriety in the place, which had come to look upon the General as a Russian nobleman of great wealth. ... He charged me ... to get two thousand-franc notes changed for him at the hotel counter, which put us in a position to be thought millionaires ...
This theme of the importance of money and the importance of being thought wealthy continues through the novel. There are frequent references to money, and it has a major role in Polina's romantic relationships.
Finally we have, as discussed by the two previous contributions, the theme of obsession. Not only are the characters addicted to gambling, with every win feeding their addiction and every loss making them think they need to gamble more to make back their losses, but even love appears obsessive and unhealthy.
In The Gambler, Dostoyevsky shows a couple of major themes.
The first two and most present themes deal with gambling itself - the addiction, and the thrill of "the bet." In his own life, Dostoyevsky was actually under very strict deadlines to finish the book so that he could pay off his own personal debts. He paints a picture of psychological pleasure when placing bets, and the torment that comes with losing them. His addiction to gambling comes, ironically, as a result of trying to move up in social status in Russia. In the end, it results in the opposite, leaving him in ruins.
That brings us to another theme - "loss." The main character has issue dealing with and accepting loss, so he continues his downward spiral.
Another theme you could mention would be "temptation." The only reason that Ivanovich gets into debt is because he is trying to court Polina. His desire for this affair, and also his inability to stay away from roulette, show the temptation he must deal with. This always ties back with "addiction."
One of the most obvious themes of the novel The Gambler is the theme of the power of addiction and lust. Through his addiction to gambling, the protagonist starts to lose himself. It also brings the theme of social status and how money and possession seems to greatly affect it. The novel also seems to hint at the danger of impulsive decision and behavior.