Chefs add sugar to tomato gravies to balance the acidity of the tomatoes, enhance the overall flavor profile, and create a smoother, more palatable sauce.
Tomatoes naturally contain acids, such as citric and malic acid, which can sometimes result in a gravy that tastes overly tart or sour. Adding a small amount of sugar helps to neutralize this acidity, creating a more balanced flavor. The sugar doesn't necessarily make the gravy sweet; instead, it rounds out the sharp edges of the tomato flavor, allowing other ingredients and spices to shine through. Furthermore, sugar can enhance the Maillard reaction during cooking, contributing to a richer, more complex flavor and a deeper color in the gravy. It also helps to create a smoother texture by binding with the pectin in the tomatoes.
Start with a very small amount of sugar (about 1/4 teaspoon per cup of gravy) and taste as you go. You can always add more, but you can't take it away! Overdoing it will result in a gravy that is noticeably sweet, which is usually not the desired outcome.