There is no statistic for us to use in proving on this, but if you think its something of a necessity, then we may try to start one here and conduct the research in finding out by engaging the bitcointalk community gamblers, it can come in any form, you may create a poll or any other similar pattern to extract the data from here, but going by the general approach, it is very difficult to establish on this, since not everything we know about gambling as well as the gamblers.