I don't know why people say bitcoin is gambling. Its not gambling. You are investing your money and time. You use your internet. So its not gambling dealing with it.
Bitcoin is a currency. You cant say money is gambling itself. But if you invest your money and convert it into bitcoin, now it is gambling. Because you take a risk investing your money to a volatile currency hoping it would rise up in the coming days, but you know it might also drop. There is still win or lose in it, so basically its still gambling. The process is gambling, not bitcoin itself.