In a large measuring cup, combine the pan drippings with enough chicken broth to yield 6 cups of liquid. Place the flour in a medium bowl and whisk in 2 cups broth mixture.
Heat the roasting pan over medium heat. Pour in 2 cups broth mixture, scraping up the browned bits. Whisk in the flour-broth mixture and bring to a boil, whisking constantly.
Whisk in the remaining 2 cups broth mixture and cook, whisking, until boiling and thickened, 7 to 8 minutes. Season with salt and pepper. Makes 6 cups.