A
hobby is an activity that you enjoy doing in your spare time.
Synonyms: pastime, relaxation, leisure pursuit, sideline
Source:
https://www.collinsdictionary.com/dictionary/english/hobbyBased on that definition I think gambling can be considered as a hobby though money is involved but as long as you are having fun and responsible on it it is all good.
No matter the true definition gambling is hobby as long as it has a form of fun in your spare time and sometthing that you enjoy regardless of final outcome.
At the moment when you relay on gambling as a source of income or additional source of money it becomes much more serious, you put much more effort and time in it and it can't no longer be defined as a hobby.