Gambling is not a way to grow your bitcoins , its a way t entertain yourself , it is legal till you are using it for fun and becomes illegal if you are involved in high money making.US bans much sites which takes huge amounts to bet.
Example: williamhills are now banned in US.
Yeah gambling is just a waste of time as it involves risk and you cannot win everytime and end up loosing at the end of the day so I don't think that this is the right way to grow your bitcoins I would rather prefer to go for trading with a good strategy to grow my bitcoins.
trading and gambling same have high risk. it all depends on your skills, both are the best way in my opinion. but gambling more to luck.