We're entering sorites paradox(1) terrain here. I would say that what differentiates "gambling" from "investing" is the amount of risk you're choosing to take. The lowest risk you can take is distributing your savings between money, stocks, bonds and real estate. For the stocks part, the lowest risk strategy is buying index funds.

So, if you distribute your investment and use index funds for stocks, I would definitely say you're not gambling by any reasonable definition of the word. In between, I'll leave it to everybody to decide for themselves where they want to put the threshold.

1) https://en.wikipedia.org/wiki/Sorites_paradox