Google Pay does not share your actual card or bank account number with the casino. Instead, it sends a one-time encrypted code for that single transaction, so a data breach at the casino's end cannot expose your real payment details.
Most Android phones also require a PIN, fingerprint or face unlock to confirm a payment, which adds a second layer of protection if your phone is lost or stolen. As with any payment method, pick a casino with a valid gambling licence before you deposit.
If you would rather use a method that also handles withdrawals, MuchBetter works a similar way, loading funds onto a secure prepaid Mastercard in the app. If you would rather not link a card or bank account at all, a prepaid voucher like Paysafecard lets you top up with cash instead.