Are there any statistics on how gambling with Bitcoin (and crypto generally) develops? I would be interested if it increases or decreases. From time to time there are articles on media like Coindesk but I'm not sure if you can believe the data.
If you are looking for the traffic statistics then you can launch your own project to track the data because traffic statistics are available in the internet. I see someone has already pointed out the data based on bounce rate.
However, if you are looking for the crypto gambling data based on the amount spend or bankroll details, then you will be disappointed because majority of the gambling websites won't provide any such data to keep themselves safe from hacking attempts.