You don't officially have to be a registered nonprofit or a charity to call yourself dot org-- but a gambling site seems a bit over the top :) Your type-ins will never, ever remember that it isn't dot com. Or they'll go off snickering. "A gambling site that doesn't expect to make any money? I'm there!"
biz on the other hand just sounds trashy. Sorry, but it does. Can't even think of a big-name established site with dot biz at the end.
This is assuming you don't have any tribal affiliation. I could see an Indian casino calling itself dot org with a straight face.