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.