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.
It's only when you invest in a gambling site you might consider gambling as investment. No way should you think gambling can be an earning source. People who think like that end up losing everything. And by the way even investing in a gambling site might be risky so your profit after certain time period is not guaranteed anyway.