Place tomatoes, water, and olive oil in a blender or food processor; blend until smooth.
Transfer tomato mixture to a large pot.
Stir sugar, garlic salt, and vinegar into tomato mixture and bring to a boil. Reduce heat to medium-low and simmer until thickened, 1 to 2 hours, stirring occasionally.
Pour tomato sauce into a clean blender or food processor no more than half full. Cover and hold lid down; pulse a few times before leaving on to blend. Puree in batches until tomato sauce is very smooth.
