I looked it up....
From the Catholic Dictionary (1958):
BETTING AND GAMBLING: Betting is in itself lawful provided that the subject matter of the bet is not sinful, that neither party is certain of the event, and that both parties understand the bet in the same way. Gambling on games of skill or of chance is lawful provided that both parties are willing to play, even though one of them realizes that he has no chance of success; that cheating and fraud are absent; and that the money staked is not required for payment of debts or to support themselves and their families. Gambling on the stock exchange is lawful unless unjust devices such as causing an artificial rise and fall of prices are employed. Betting and gambling are dangerous, for they easily lead to sin, misery, ruin of self and others. In 1590 Pope Gregory XIV found it necessary to restrain the sporting Romans, by forbidding, on pain of excommunication, all betting on the results of papal elections, the duration of conclaves and the creation of new cardinals.