If they were really in a relationship, I don't think it'll be possible for the man to have completely hidden that he has been gambling all these years because some habits are really difficult to hide, and men really suck at hiding things unless you as the woman are not observant enough or not paying attention.

Marry your type; there are women for whom gambling is not a deal breaker for them. go for them, so even if you want to be silly and secretive about your gambling habit, it will not ruin your marriage when they find out.