But to say that a doctor could prescribe gambling for a patient is something that be wrong and unethical for they profession, because advising patient to gamble may result into bad gambling experience.
Of course, other alternative games can be done instead of gambling to relieve stress. But if the patient really prefers games that have bets, gambling could be a solution too.
If it is used as therapy, its use or activity should be monitored. it cannot be excessive and it must be supervised. I'm not sure any doctor would advise that. There are many ways people can get peace from the stress of work.