The Gemara in Eruvin, Daf 82 discusses the issue of whether a gambler is qualified to render testimony according to Halacha. Halacha disqualifies thieves from serving as witnesses, and so the question becomes whether or not earning money through gambling constitutes thievery. The Gemara cites a dispute on this matter between Rabbi Yehuda and the Chachamim. Rabbi Yehuda is of the opinion that "Asmachta Kanya," meaning, since both parties gamble with the understanding that they might lose their money, and they accept the terms of the game, the winner legally acquires the money from the loser. Gambling would thus not involve theft, and gamblers would be qualified to testify. The Chachamim, by contrast, hold that "Asmachta Lo Kanya" earning money in this fashion, where the wagers are placed in the hope of winning and the loser surrenders his money halfheartedly, does not constitute a legal acquisition. Therefore, earning money through gambling is considered theft and disqualifies the gambler from testimony.
Tosefot (Medieval French-German Talmudic commentators) follow Rabbi Yehuda's position, that gambling does not constitute theft. Nevertheless, Tosefot would agree that full-time gambling, as one's primary occupation, is forbidden. The Rambam, on the other hand, appears to hold a different view. Although he seems to accept Rabbi Yehuda's opinion, he views gambling as theft Mi'de'rabbanan by force of rabbinic enactment. While Torah law would consider earning money through gambling a legal means of acquisition, the Rabbis legislated that it should be treated as theft. According to the Rambam, then, gambling is proscribed by force of rabbinic enactment.
In truth, even Tosefot's position would not necessarily sanction gambling. The Rivash (Rabbi Yitzchak Ben Sheshet, Spain - N. Africa, 1326-1407) writes in one of his responsa (listen to audio for direct citation) that even if gambling is not strictly forbidden, it is nevertheless a "Davar Mechu'ar" a loathsome activity. He therefore rules that if a person uttered a vow to abstain from gambling, the Bet Din (rabbinical court) should not annul the vow for him. (Normally, Bet Din is empowered in many situations to annul an individual's vow so that he would no longer be bound by it, a process called "Hatarat Nedarim.") Given the impropriety of gambling, even if it is not forbidden according to the strict letter of the law, Bet Din should keep the vow in place, rather than having it annulled. And although the Shach (commentary on the Shulchan Aruch by Rabbi Shabtai Hakohen, 1623-1663, Europe) disputes the Rivash on this particular point, all would agree that gambling is not an appropriate activity for a Jew.
Thus, according to the Rambam gambling is forbidden according to Halacha, and according to others, Halacha outright forbids only full-time gambling, but nevertheless frowns upon even occasional gambling.
It should be noted in conclusion that a Jew is to spend his free time involved in meaningful activities such as Torah study, Mitzvah performance, acts of Chessed (kindness), and so on, such that there should be no time for wasteful activities such as gambling.
