How is it "gambling" any more than, say, stock is? Because stock prices vary based on actual real events? Well, bitcoin prices vary based on actual real events as well.
Buying stock is buying interest in a company. Buying an individual stock certainly does have a significant speculative element, but in aggregate companies are expected to make money, and so stocks are expected to either increase in price or pay dividends.
Buying bitcoin as an investment is more like forex trading (in fact, that's almost literally what it is), which is zero-sum.
I would argue that buying a single stock is gambling, but buying into a fund, or taking the time to research and invest in hundreds of stocks, is investing.
Buying bitcoin as an investment is more like forex trading (in fact, that's almost literally what it is), which is zero-sum.