Heat the oil in a medium saucepan over medium heat. Stir in the flour and cook for 1 minute, then add the chili powder and cook for 1 more minute. Gradually stir in the water, mixing well with a whisk to make sure you get out all the lumps, then stir in the tomato sauce and season with salt and garlic powder to taste.
Reduce heat to low and simmer for 10 to 15 minutes. (Note: If sauce thickens too much, just add a little water to thin it out a bit.)