Grate zucchini, place into a clean tea towel and wring out excess liquid. Set aside. Add beef to a large bowl, add zucchini and rest of meatloaf ingredients to the bowl. Gently combine.
Preheat grill for medium-high heat and indirect grilling.
Place meat mixture onto a flat cast iron griddle and form into a uniform loaf. Place onto grill and cook for 60 to 90 minutes. Look for an internal temperature of 165 degrees F.
Heat oil in the pan and cook sliced onion and mushrooms over medium heat. About 5 minutes. Season with salt and pepper. Cover to keep warm.
In a small saucepan, bring 2 1/4 cups of water to a simmer, add Better than Bouillon, and stir to dissolve. Simmer for 1 minute. Remove a ladleful of mixture and add to bowl with xanthan gum. Stir well. It will clump quickly, add more broth as needed. Add to pan and whisk through. Let the mixture simmer for 2-3 minutes over medium heat, whisking often. Once thickened, strain gravy into a serving bowl. This will remove any additional clumps from the xanthan gum.
To serve, carefully cut meatloaf into 6-8 slices. Serve a few tablespoons of gravy and about a 1/4 cup of mushroom onion mixture.