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.
Even if you invest money on any gambling bankroll it is not a gambling but you're still investing and it is just a different way of investment. Gambling games are just to have fun and not for earning money instead we need to spend money to play these games. So investing and gambling both are completely different not same.