Preheat oven to 350F
Preheat a large skillet over medium heat. Add butter and melt.
Add diced onion and celery to the skillet. Cook for 5-7 minutes, until onions and celery and soft and translucent.
Add diced apple to the skillet and cook an additional 3-5 minutes, until apple just begins to soften.
Remove onion, celery and apple from the stove top and add to a large bowl. Stir in dried gluten-free bread cubes, sage, salt and ground black pepper.
Pour chicken broth over the bread cubes. You may not need all of the broth. Add about 1/2 cup at a time just until the bread is moist and not soggy.
Pour the bread mixture into a 9x13-inch baking dish. Dot with the remaining two tablespoons of butter.
Cover the baking dish with aluminum foil. Bake at 350F for 30 minutes. Remove the aluminum foil and bake for an additional 10-15 minutes, until the top is a light golden brown. Let cool for 5-10 minutes before serving.