As we know that most gambling sites will require you to gamble and wager your whole balance once, before you can cash out or withdraw from the gambling site and they take this measure so people do not do money laundering whereby deposit and withdraw without gambling.
Now the point here is that on the gambling site, there are many easy winning bets with odds like 1.01x etc. If the money lauder is sensible he can easily be wagering the whole amount by placing a few bets where the odds are very low and the winning chances are 99%.
So does this mean that there is a loophole here and the money launderers can easily by pass this wagering requirement?
Gambling sites are very aware of the possibilities and casinos have been traditionally targets for this activity, so it definitely does not go unnoticed by anyone. Money laundering with bitcoin is actually not generally possible in the way you're describing, as the actual wallets are disconnected from the balances when bets are taking place. The casino will often have control of the player wallet, even pooling lots of them together, but keep track of balances that the player is allowed to withdraw by a different mechanism. If a player deposits money, it doesn't move back and forth between wallets when under control of the casino, but rather it might be "balanced" or have funds paid to the casino at the end of the day, otherwise it would go out the same route it came in.