It sounds like one of those irregular verbs; I invest, you trade, he just gambles.
In a sense buying or selling shares, indices, currencies or commodities or whatever else is always gambling, as they may go up or down in price. Most 'investors' would not want to call it this though, as gambling is BAD. But even with the odds in your favour you can still lose on any one punt.
However, if the odds are in your favour, even by a small amount, in the long run if you keep 'gambling' eventually you will win, which is how Las Vegas was built (by the casinos of course, who have an edge, not by their customers).
So the argument is all semantics really, and what you understand investing and gambling to mean.