If a gambler has already made an account on such a casino site and if he has already received a bonus, then if he changes his Gmail and creates an account later, he will not be able to withdraw the bonus he will get, but he will have to gamble with the money from that bonus and if he can collect a good amount of money by gambling, then he will be able to withdraw. However, I think that very few gamblers will make accounts on all these gambling sites with multiple Gmails just to take the amount of money that is given as a bonus. Just like I do not use multiple accounts for gambling, but rather have only one account on a reliable casino site and I try to continue gambling with that one account. Maybe those who have thoughts like me also do the same thing.
Creating a multi-account for the sake of welcome bonuses is a pointless idea, because, as you said, almost all gambling platforms set wagering requirements for bonuses, and as a rule, these requirements are too high, which makes the pursuit of bonuses pointless both for the average user and for the avid abuser who has many accounts.