I think most of the people are already using crypto for gambling. At least those who know about cryptocurrencies use crypto for gambling. It is only those who do not know about crypto that are still gambling with fiat money. Or maybe the gambling casino where they gamble do not offer crypto payments and therefore the only option for them is to gamble with fiat. Also, the bookies who operate offline still use the local currency as the mode of payment.
Some gamblers who had funds on their e-wallet instead of buying cryptocurrency to deposit opted to deposit directly.
Some e-wallets in our country charge as high as a $15 transaction fee on cryptocurrency, whereas their e-wallets offer almost free transactions. For convenience and to avoid higher transaction fees, users opt for their fiat-based e-wallet.