You can cut back on food to the necessary deficit and start losing weight without going to the gym. Or you can eat chocolates and work off more than you ate in the gym. In either case you will lose weight.
But what exactly will lose weight, the waist or buttocks, no one knows. The result is not predictable. In both cases. And what to do after losing weight to maintain the result? This, unfortunately, is not written in the recommendations of any diet.
As the saying goes, "When you lose weight you'll realize that you weren't loved not because you were fat, but because you were stupid!" I found a deep meaning in this joke that has to do with the process of losing weight and the state you come to as a result.