I have been where you are now. My husband is a (now in recovery) compulsive gambler. We stayed together, but it has been a very very hard road, one which I don't think I'd take again, even though we now have a good, loving relationship.
Firstly, CGs are masters of deceit. They deceive everyone around them, and they deceive themselves. They are manipulative in the extreme, and they have completely warped views of the nature, value and mechanics of money. Him laughing at you is exactly the sort of thing that my husband would have done in the past. This is because he truly, in himself, cannot see that £500 wasn't his to use, and is a large sum of money. Gamblers can win and lose such huge sums so easily, especially online. They lose the ability to understand money at all, it is only about the thrill of putting on the bet.
The awfully sad thing is that it literally is all they can think of, and they will do anything in their power to keep the money going. My husband told me that, when he was gambling, if he had a phone call to say I'd been in a car crash while he was in the bookies, he'd still stop to put the bet on.
Recovery is so hard. The gambler has to make the decision for himself and has to agree to complete and utter openness. He also has to acknowledge the hurt and destruction he has created, and allow you to talk about it openly with him. This is really hard. My husband had three of four attempts - but each time he didn't "really" want to stop. So he didn't. He stopped for a bit, struggled, blamed me, started again. It was truly the worst time in my life.
I did a lot of reading and realised that he had to be forced to a "rock bottom" state, so I threw him out. He had absolutely no where to go. His parents had turned against him because of the behaviour the gambling caused. He owed money to his friends. His credit had run out, even payday loan companies wouldn't lend to him. He had nothing but the clothes he stood up in, and (thank god) his job.
He is now back, and is gamble free for nearly a year. He still can't handle money, or have access to the bank account. I give him cash when he needs it. He shows me receipts. I check his credit file monthly. I know that if he wanted to gamble, he could, despite all this. There are other things that convince me things really have changed. He is completely open with me, on all levels. He has a sponsor, who he checks in with regularly, but hasn't found group meetings particularly useful (I think this is a personal thing - they work for others).
So, I've got my husband, but I don't know when if ever he will learn a normal relationship with money. I've had to accept this. I've struggled to let go of the hurt and betrayal and complete blinding fury around how he behaved and what he did. The only way I've been able to even start is because he's faced up to it completely. If he lies to me again, about anything at all, that is it and he knows it.
But, would I do it again or recommend it to anyone else? Not a chance. It is a hard hard road, and mine may yet end with him relapsing and our relationship ending. Gambling is so unbelievably destructive to those who are compulsive. I was totally naive about it until it him me full on in the face.