If you are an oil company entering the futures market to reduce your risk on oil prices, you aren't gambling. If I enter the market and short oil for fun, I'm gambling.
The stock market isn't gambling if you are just buying equities - you are literally purchasing a small stake in a company. The fact that prices move around a lot doesn't make it gambling per se. If I offered a new price on your house every day, it doesn't make your house purchase a gamble.