In a large saucepan, heat the fat you have reserved and butter (8 tablespoons total) over low heat. I like to use half reserved fat and half butter.
Slowly whisk in your 4 cups of reserved turkey stock from the drippings. If you only have 2 cups of turkey stock add 2 cups of chicken broth into the pan, constantly whisking.
Cook gravy over medium heat until it thickens. Add pepper. Add salt to taste. Remember gravy will thicken as it cools. Add more chicken broth to make it thinner if desired. Store in the fridge.
TOO RUNNY: Roux-thickened gravies continue to thicken as they cool, so keep that in mind. If really runny, make a slurry with cornstarch or flour, and whisk into gravy.
TOO LUMPY: To fix lumpy gravy, pour it into a blender and blend for 30 seconds.
TOO THICK: Whisk in some turkey or chicken stock until you get the desired consistency.
TOO BLAND: Season with thyme, sage, bay leaves, salt, and pepper. Bring to a simmer and remove the bay leaf before serving.
HOW TO STORE GRAVY/MAKE AHEAD
If you want to make this ahead of time, store in the fridge. To use, heat over medium heat and add chicken stock to thin it out as needed.