Strictly speaking, gambling is when you chose to play zero-expectation or negative-expectation games, that is, games in which you have no advantage. This describes all casino games with the possible exceptions of poker (if you are the most skilled player at the table) and blackjack (if you can successfully count cards without being detected by the house).
Trading becomes gambling for those who don't have a validated trading plan. Except for your broker and his ubiquitous commissions, there is no house you're playing against. Therefore nobody with a guaranteed advantage against you. Small fry can compete with the big fish if they are smart enough and determined enough to succeed. That's why I love trading.