Is gambling OK? Don't bet it! There's not an easy or instantly obvious proof text answer to that question.
In my own point of view, gambling is not a sin if you can control yourself when you play and if you are not an addict in gambling because when you only play, you only play on it not making it as a hobby that will come up being addicted on it. That's why it is better to limit yourself when you are playing.
This is the best if anyone can follow this. But I am sure knowing the human and greed of money this will be not easy for those gamblers who just start for fun and since they mostly would not have that self control it would be difficult for them to quit at their ease. Because money is what changes their mind and easily get into the gambling addiction mode.
This is what usually happen even a certain gambler would set a particular limit to himself in his mind that he would stop if he is already on greens or profits but our greediness would really break those limits on which most people do really fail because of this one. Gambling isnt a sin as long you do know on what you are doing and your limitations when you engage on it.
If you set realistic targets and limits, greed is overcomed in such an instance because you will only follow through on the plan but when you have no plan that's when you have second thoughts about whether you can win some more or not.
I honestly have not come across a statement in the Bible saying that gambling is a sin. Sin, as for me, is subjective and is highly dependent on ones moral upbringing. What may be acceptable to you may not be acceptable to others. That is basically it. However, I do recognize the importance of relating sin to that found in the Bible, the latter being the fundamental basis of what is right from wrong.
But in relation to where your question is leading, I think gambling, in a more general sense, is not a sin because it is part of a business's mechanism to know if you are indeed profiting. Otherwise stated, you gamble by investing a certain amount on the premise that your investment will grow within your desired time.
Gamble is not really just focusing on the the casual gambling game itself but there are decision in life that do really needed to be gambled for you to earn something which means its not totally a sin because you are accepting the risk to earn something the thing here is that you should really have the control of yourself and you are prepared on the possible circumstance ahead regarding in your decisions have been made.