An important question: An unstuffed turkey should be cooked until it is at least but not much more than 170 deg F. For a 21 lb turkey this will range from about 4 1/2 - 5 hours at an oven temperature of 325 deg. One of the best places to check the temperature of the turkey is under the leg. Also it is not ok to leave lumps in the gravy

You strain them out with a fine mesh strainer. Once the starch in the flour has gelatinized you cannot make it smooth if it is lumpy without simply removing the lumps.
Use a cooking screen, or a filter.