Is gambling OK? Don't bet it! There's not an easy or instantly obvious proof text answer to that question.
In my own opinion, gambling is not a sin if you are just playing gambling for only entertainment or fun, but if you take gambling as a scheme to earn profit it is a sin because you will get greedy in playing and you might lose all of your money in gambling that amy ruin your life.
In that regard it doesn't matter whether you win or lose. Thinking all the time about earthly things, like making money with gambling for example, considered sin by most religions.
And you said it right about gambling for entertainment purposes. Since it's for fun only it can't be a sin, no one forbids to have some fun from time to time. That's just my opinion, of course, I know some religious people wouldn't agree with me.