Investing in a ponzi is gambling as you are investing into something in which the outcome/result is unknown.
Putting money into a ponzi isn't an investment, then, is it?
It would only be classed as a legitimate gamble if you were informed prior to 'investing' that it was a ponzi and that there was a risk you would lose your money. Even then it would still need to operate transparently as a ponzi so that all participants were fully aware of the mechanisms of the 'game'.