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.
You can never be sure that you will get back what you have invested...
In this case, gambling is much more transparent. You instantly see if you won or lost.
Gambling will give you instant pleasure with your money, but investing takes the time to show the results. Investing is a better option if you're looking for good returns from your money, but the only thing is you should find a good trusted sites and products to invest your money.