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.
Your wrong, gambling can't be considered as a earning source because it is meant for just a fun and entertainment purpose. You should go to gambling when you need some enjoyment and if you want to grow your money then should find some good investment options, not gambling.
for regular users who have not dedicated their lives to gambling it can never be an earning source. and most people are like this, it is only for entertainment purposes. the only way you can make profit from gambling is to get lucky and that doesn't happen every day or if you are a professional gambler like those big Poker players who do it for living.