In gaming, you are trading on risks that are artificial, eg. betting on a dice roll or on a roulette spin. Those risks do not need to exist, and they risk solely for the purpose of betting.
In the derivatives markets, the risks you're trading on are not artificial, they're already existant. The purpose of markets is to transfer those risks from people that bear them and need to offload them (hedgers) to people that have appetite for them (speculators).
This debate has been at the core of these markets since the dawn of derivatives markets. See this fascinating Court Case on grain futures: https://en.wikipedia.org/wiki/Chicago_Board_of_Trade_v._Chri....
At the time, there was a lot of debate on whether grain futures were gambling. The final decision by justing Wendell was not because (paraphrasing) "sure a lot of people will speculate, but saying that these markets exist purely for speculation is off - there's a subset of participants that need to hedge risk, and that makes these markets crucial" (note: intent to deliver was the early form of hedging).
The original response:
"[T]he plaintiffs chamber of commerce is, in the first place, a great market, where … is transacted a large part of the grain and provision business of the world. Of course, in a modern market, contracts are not confined to sales for immediate delivery. People will endeavor to forecast the future, and to make agreements according to their prophecy. Speculation of this kind by competent men is the self-adjustment of society to the probable .... It is true that the success of the strong induces imitation by the weak, and that incompetent persons bring themselves to ruin by undertaking to speculate in their turn. But legislatures and courts generally have recognized that the natural evolutions of a complex society are to be touched only with a very cautious hand. .... It seems to us an extraordinary and unlikely proposition that the dealings which give its character to the great market for future sales in this country are to be regarded as mere wagers or as ‘pretended’ buying or selling, without any intention of receiving and paying for the property bought, or of delivering the property sold, within the meaning of the Illinois act"