TheFreeDictionary describes gambling as such:
"To bet on an uncertain outcome, as of a contest."
Trading, whether it be stocks, futures or forex, in my opinion is not gambling by this definition (if done correctly). The key term here being 'uncertain'.
Traders use signals such as fundamental analysis or technical analysis (chart reading) to mitigate the risk or 'uncertainty' involved in making an investment.
Another tool the investor uses to safeguard their investments is the stop loss. This is a predetermined sell price you set with your brokerage which automatically signals your brokerage to sell your position in a stock should the price drop bellow your preset stop loss.
So, a winning trader will give himself the best possible odds of success by watching a group of stocks for signals which have proven themselves to be accurate indicators of stock direction in the majority of cases. While no signal is accurate 100% of the time, there are some which are accurate 60%-80% of the time.
In addition the winning trader also uses stop losses on every trade in order to protect him/herself from major financial loss. This stop loss is so important that many well known investors hold that if you use correct stop losses on every trade, you can be wrong (or lose) on your trades 50% of the time and still make money.
One final note. If you talk to professional gamblers, they will tell you that what they do is not gambling. Like traders professional gamblers play the odds and only bet when the odds are in there favor.
It isn't a coincidence that the same handful of professional gamblers make it to the World Series of Poker every year. They aren't the luckiest people on earth. In fact they aren't any luckier than your average Joe. Their just regular Joe's who have figured out how to use signals (tells), and mathematical probability to gain an advantage over their competition.