If you are using mushrooms, saute them with butter until cooked through in a small frying pan. Set aside.
If you are using the turkey giblets, boil them in water in a 2-quart pot until they are cooked through. Chop into small pieces. Set aside.
After removing the turkey from the roasting pan, move the roasting pan to cover two elements on the stove. Or pour the turkey drippings into a sauce pan and put it on the stove.
Turn the stove element(s) on medium heat.
Heat the turkey drippings until they are boiling.
Sprinkle the flour across the top of the turkey drippings.
Stir with a whisk while cooking over medium heat for about 5 minutes. Make sure that the flour has completely dissolved into the turkey juices so that there aren't any lumps.
Gradually stir in the chicken stock.
Let simmer, stirring occasionally, until the turkey gravy is the consistency that you want.
Stir the sherry into the gravy.
Add salt and pepper to taste.
If you are using giblets and/or mushrooms, add them to the gravy and cook until heated through.
Notes
To make the gravy gluten-free, use measure-for-measure gluten-free flour instead of all-purpose flour. You will need to stir it longer to get the flour to dissolve but it will work.
If the gravy is too thin, add a little more flour and simmer for a few minutes longer. If the gravy is too thick, add a little more chicken stock.