It is quote impossible to know What percentage of bitcoin investors are gamblers. I googled this topic and finally used AI but couldn't find it anywhere. It is impossible to truck it. It is possible to track how many wallets are holding bitcoins but how is it possible to track the bitcoins you are holding? So how is it possible to find out which Bitcoin users are gambling? So I think such question is unnecessary
What makes it harder Bitcoin is pseudonymous, one person can own more than one address.
The only way to know is fiat casino, since one person can only have one account in a bank, even they use more than one banks, their names are recorded and we can eliminate it in order to get the actual statistic.
In Bitcoin, it's impossible.