Unflavored gelatin is a low-calorie food, and doesn't contain empty calories like sugar-sweetened dessert gelatins. Virtually all of the calories in unflavored gelatin come from protein, as this food has no carbohydrates and just a trace amount of fat. To make your own healthy dessert at home, mix unflavored gelatin with fruit juice instead of water, and don't add sugar.
A 1-ounce package of dry, unsweetened gelatin contains 94 calories. Considering the average non-sedentary, 125-pound woman burns 2,000 calories per day and the average 175-pound man burns 2,800 calories per day, plain gelatin fits easily into most diets. At 107 calories per ounce of dry mix, flavored, sweetened gelatin doesn't break the calorie bank either -- although it is high in sugar.