Bring 2 cups broth, neck or wings and trimmings to simmer in a large saucepan over high heat. Simmer, stirring occasionally, until all the liquid evaporates and trimmings begin to sizzle, 20 minutes or so. Cook, stirring frequently and watching carefully, until dark layer forms on bottom of the pan, another 3 minutes or so.
Reduce the heat to medium and add onion, celery, herbs, garlic, pepper, and salt. Cook, stirring frequently, until the onion is soft, about 10 minutes.
Add in wine, if using, and bring to simmer, scraping up any browned bits (if not using wine, simple add remaining chicken stock now). Add remaining 2 cups broth boil on high. Reduce heat to low, cover, and simmer for 1 hour. Strain the turkey stock through a fine mesh strainer and discard solids. (You should have about 2 cups of stock - this can be made and refrigerated up to 2 days ahead.)
Melt the butter in a saucepan over medium-high heat. Brown the butter (cook until it smells nutty and there are light golden solids at the bottom of the pan. Reduce heat to low and slowly whisk in strained stock. Sprinkle xanthan gum, gelatin, and nutmeg over the top and whisk in. Increase the heat to medium-high and bring to simmer and cook until thickened, about 5 minutes. Add any turkey drippings, if you are using them. Adjust thickness of gravy with more broth or xanthan gum, if desired. Season to taste with salt and pepper.