Place chicken breasts between 2 sheets of heavy plastic or waxed paper on a solid, level surface. Firmly pound with the smooth side of a meat mallet to a 1/2-inch thickness.
Combine bread crumbs, 1 teaspoon oregano, 1/2 teaspoon garlic salt, salt, and black pepper in a shallow bowl. Dip both sides of chicken in bread crumb mixture until evenly coated.
Heat 1 1/2 tablespoon olive oil in a large skillet over medium heat. Cook 2 chicken breasts until browned and no longer pink in the center, about 4 minutes per side. Repeat with remaining olive oil and chicken breasts. Arrange chicken breasts in a 9x13-inch baking dish.
Combine tomatoes, sugar, 1 teaspoon garlic salt, basil, 1/2 teaspoon oregano, salt and black pepper in a saucepan. Cook over medium-low heat until heated through, 10 to 15 minutes. Pour sauce evenly over chicken breasts in the baking dish.
Cover tomato sauce with pepperoni and mozzarella cheese.
Bake in the preheated oven until cheese is melted, 20 to 30 minutes.
