Is gambling OK? Don't bet it! There's not an easy or instantly obvious proof text answer to that question.
Gambling is certified as sin, because most people who get involved here is became greedy but not all of them.
Because there are some gamblers played for fun only.
Where did you read that it is a sin, when you say certified it is like there is an official statement saying in general that it is a sin. I doubt if it is applicable to all if there's any, in our country some illegal gambling are prohibited but I don't believe it is prohibited because it's a sin.
Moreover, please do consider that there are so called legal gambling which is supported by the government or operated by the government like a lottery in our country.
I bet that you don't know the book that we called "Bible", it is where you could read that gambling is a SIN, that our lord God is prohibits us to do. But we are always violating most of his rules, and we are just obeying ourselves. If the Government said that it is illegal then most of us are not going to do gambling anymore, but when it is God who said that it is a Sin and shouldn't be done by anyone in the first place, we didn't listen.
So, in short, it looks like that we didn't fear God, instead we fear to the people who have position in the Government.