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.
Not all investments come with fixed profit assurances like investing in share market, bitcoins, casino house bankrolls etc., All these investments come with some risks, and in the longer term, we have better chances of making profits from these investments. If you want a fixed profit, then only one can give FD and interest rates will be very small.