Bake Grain-Free Planet's gluten-free "bread" in advance per package instructions. Once you have a baked loaf, cut the bread into small squares (1/4", the smaller the square, the better). Set aside.
Heat oven to 375°.
Score chestnuts (one slice on the outer shell will do). Place chestnuts in a small pot with cold water. Bring to a boil. Once the water reaches its boiling point, turn down to simmer and place a lid askew on the pot. Cook for 15 minutes. Or, simply purchase a jar of boiled chestnuts and mince finely. In the meantime, heat 2 tbsps olive oil in a large stock pot over medium-high heat. Chop onions, celery, mushrooms. Add vegetables to the stock pot along with cranberries and cook for ten minutes. Stir occasionally, coating the vegetable mixture with olive oil.
Once chestnuts have cooked, strain and peel off the outer shell. (You can skip this step if you purchased jarred chestnuts). Chop chestnuts finely and add into the vegetable mixture along with minced garlic. Chop fresh herbs and add to the mixture as well. Stir all ingredients together along with dried herbs.
Add baked bread pieces into the vegetable mixture. Stir and season with salt and pepper to taste.
Transfer mixture to the cavity of a turkey or a casserole dish to bake independently.
If baking independently of a turkey, bake for 30 minutes (also at 375°).