To gamble is to decide on possible results in which would likely be on two or more choices for a result or even an uncertain result.  Since sin was mentioned then some of us would likely link this with religion which could be a sin or would not be.  It is like doing something with multiple possible results and leaving it all in the hands of God to whatever the result will be.  But if you say sin in the laws of the land then it could probably mean doing wrong for violating a rule of law in which depends if it is legal or illegal to gamble in that country.