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.
I've never made use of any e-wallet as a means of either buying crypto or depositing into a casino, so I guess I can safely say that I have never been charged this high for a transaction.
Over here in my country, banks do not allow direct conversion to crypto currency, so we go the p2p way with zero fees, you send the amount of crypto you want to sell to the buyer and the buyer send you the agreed amount to your bank account, this is how it's been over severals years now and I think I like it as it completely shuts the government off.
So if I want to play or gamble using cryptocurrency when I don't have any, it's easy to convert fiat to crypto easily without paying any fees, the deposit to the casino and play.
But if I want to play using fiat (which I hardly do anyway), I simply deposit the money directly to the casino if the casino supports my local currency.