My wife and I agreed that 5% of my monthly wages will be on my own discretion meaning i can do anything from that.
Your wife thinks normally, indeed 5% of husband's income is a decision that must be made, sometimes I've also heard someone tell me of his 100% income, he only takes 20% for himself.
So, I want to ask you, your wife knows you gamble and use that 5% budget....
Because I have a friend caught his wife gambling, he only gives 50% every month to his wife, 50% he spends on betting in the end, it's a mess.
I think the 5% he spends at his discretion is basically anything he wants, he can do anything with that amount. The daily needs and other essentials must be managed from that 95% and hence the wife has allowed him to spend 5% without asking where it goes.
You're right though that once the wife gets to know that the 5% is going towards gambling, it might be a mess because gambling is something wives consider bad for whatever reason.
I feel ironic that we husbands take 5-20% of our own money and yet feel worried about the wife not being upset of how we spent it.