Gambling can never be considered as a investment, because with investment you have assured pay back after certain time period but with gambling nothing is assured. So in my view gambling can be considered a earning source and not as a pure investment.
Your wrong, gambling can't be considered as a earning source because it is meant for just a fun and entertainment purpose. You should go to gambling when you need some enjoyment and if you want to grow your money then should find some good investment options, not gambling.