While gambling is not ennobling and there are perhaps better uses of our time, there is nothing inherently immoral about gambling provided it is done responsibly and in moderation. I would prefer someone gamble than, for example, cheat on an exam. I reject gambling as a way to earn a living because then one is not engaged in making a better world.
Argument: Gambling is immoral because one could become addicted to gambling.
Response: By this reasoning, drinking wine is immoral because one could become addicted to alcohol. But obviously, not everyone who drinks a glass of wine with dinner becomes an alcoholic. Drinking wine may be immoral for other reasons, but the mere possibility of addiction isn't one of them.
Argument: Gambling is immoral because it is not essential to living.
Response: Many things are not essential to living. That doesn't render them immoral. By this reasoning owning a boat is not essential to living, and therefore owning a boat is immoral. Obviously, however, this is not true.
Argument: Gambling is immoral because it is "usury," that is, using money to make money.
Response: If "money making money" is immoral then so are savings accounts, the stock market, and any other investment vehicles where our money "works" for us.
Argument: Gambling is immoral because it is appealing to the unknown rather than trusting God.
Response: The Apostles appealed to the unknown in casting lots to decide who would replace Judas Iscariot.
We can say that we just don't like gambling and we would prefer that people do not gamble, which is fine, but let's acknowledge that our mere dislike of gambling is an emotional response and is not a moral argument. And none of the arguments above are satisfactory.