When it comes to gambling one should not hide anything from their spouse, because in time of help it will be difficult for the spouse to help, not being able to figure out the direction where the problem is coming from.
Truth and trust is most important in any relationship we find ourselves, we should learn how to open up to ourselves.
OP did not tell anything that the guy hid something from his second half. For example saying I will go to gym, and instead go gambling is one thing. And saying I will go have some fun, have a rest (and if she hasnt questioned what he plans to do or where he is going) is totally different. If not asked, not be first to tell does not mean keeping something in secret
As you have talked about trust, then this chick has failed with trust. Guy has been doing something, she found he has been doing something without her knowing or controlling it, and she is the one who has lost trust.