Wow that's tough. I would be very upset if my FI was hiding a 5k debt from me and used money that was given to "us" for our wedding to pay it off. I do however agree with Sammysgirl and that he probably was ashamed of this and this is why he did not want to tell you or anyone else about it. I don't think you should make any rash decisions right now, but try to find some understanding in this situation. If he does have a serious gambling problem then that is something he needs to find help for, especially because that ultimately effects you and your relationship.