It's certainly possible to build a successful site on a different TLD.
However, you can be certain that your promotions will bleed some type-in traffic to the dot.com. If it's just a parked page the user would realize their error, but if there's a real site on the dot.com you might never get another chance at that user.
The same would go for domain names with hyphens, if you don't control the version of the name without hyphens.
I'd encourage you to look for a different name where you can get control of the dot.com for yourself.
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.