How can someone become a legend in gambling if he's not going to become succesful with it?
The closest answer is when they break new records in gambling.
There are a lot of professional gamblers out there that is a "legend" and yes, like the usual gamblers they already lost a lot of money in gambling. Successful as a gambler since they are known gambler and not successful as a gambler because they lost a lot of money.
if someone becomes successful in gambling, is it an achievement or not?
It is, people know you for that certain achievement. As I said, you will become a successful gambler if you break records in gambling.