Is gambling always a sin according to Catholicism?
The short answer is no.
In its moral aspect, although gambling usually has a bad meaning, yet we may apply to it what was said about betting. On certain conditions, and apart from excess or scandal, it is not sinful to stake money on the issue of a game of chance any more than it is sinful to insure one's property against risk, or deal in futures on the produce market. - Catholic Encyclopedia
There is at least one patron saints of gambling: St. Cajetan.
St. Cajetan (known as St. Cayetano in Spanish-speaking countries) is the Patron Saint of Gamblers. Cajetan, born in 1480, was a lawyer and the son of a wealthy family. Driven by a desire to reform the Church, he traveled to Rome, became a priest, and founded a religious order. He used his family fortune to create hospitals that served both the physical and moral needs of the poor. He also established pawn shops and credit unions to provide loans to the poor.
Cajetan's connection to gambling is obscure. Popular lore says the people would ask him for a favor, and bet him a rosary that he couldn't come through. Since he always came through, he was able to get people to pray more. Based on what I know of saints, this feels like a pious retrofitting of a reason to a patronage, and it just doesn't sound likely.
It's more probable that his loans helped people get out from under the predatory interest rates of loan sharks, and many of the these loans were the result of gambling debts. Then, as now, a compulsive gambler could destroy his family, so it seems more likely that Cajetan probably helped problem gamblers get back on the right path both financially and morally. - Who Is The Patron Saint of Gaming?