It's actually not. To be gambling there has to be a certain level of chance involved. In disc golf (like other sports) you have control over the outcome through your performance.
Guess it depends on your point of view, but there's always a level of chance even in disc golf and other sports. If there wasn't an element of chance involved, then if you play the absolute best round of your life, you would win. But even that is no guarantee of victory, since regardless of what you do, your opponent can still play better than you.
The dictionary definition of gamble is "1. a) to play a game for money or property, b) to bet on an uncertain outcome or 2. to stake something on a contingency : take a chance". Most tournaments are structured as an organized bet. Like when you play poker, the only money on the table is from the other players, in disc golf, the bulk of the "purse" is derived from entry fees. You're buying in for a chance to win a portion of the collected entries. Technically, some states might call what we do illegal but we're far enough under the radar to get away with it.