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.
I think you misunderstood about this thing though, gambling is sure like investing though but with higher risk to get back what you desire. It is not always that you can get your money because of the risk but if you can outcome that risk you will get more better payout mean while if you only investing with some small amount I dont think that you can get some penny from investment