Preheat your oven to 300°F (150°C). Cut the GF bread into 1/2-inch to 1-inch cubes. Spread them in a single layer on two baking sheets. Bake for 20–30 minutes, tossing halfway through, until the bread feels like a crouton—hard and dry, but not browned.
Increase oven heat to 350°F (175°C). In a large skillet or Dutch oven over medium-high heat, cook the sausage. Break it up with a wooden spoon until browned and cooked through. Use a slotted spoon to transfer the sausage to a very large mixing bowl, leaving the rendered fat in the pan.
Add olive oil to the reserved sausage fat in the skillet, if needed. Add the onion and celery. Sauté for 8–10 minutes until softened and translucent. Add the garlic and fresh herbs (sage, rosemary, thyme) and cook for 1 minute until fragrant. Season with a pinch of salt and plenty of black pepper.
Pour the vegetable mixture over the sausage in the large bowl. Add the dried bread cubes and the parsley. Toss gently to combine
In a small bowl or measuring cup, whisk the chicken stock and the beaten eggs together. Pour this mixture over the bread/sausage mix. Fold everything together gently. Let it sit for 2 minutes. If it looks too dry, add a splash of more stock. It should be moist but not swimming in liquid
Transfer the mixture to a greased 9x13 baking dish. Cover tightly with foil and bake for 20 minutes. Remove foil and bake for another 20–25 minutes until the top is golden brown and crispy