Combine the soy sauce, brown rice syrup, garlic, ginger, and pepper in a small bowl and mix well.
Prepare the grill, and preheat to 375°F.
Place the mushrooms stem side up on a baking dish. Pour the marinade over the mushrooms and let marinate for 1 hour.
Pour the excess marinade off the mushrooms, reserving the liquid, and place the mushrooms on the grill. Grill each side for 4 minutes, brushing with the marinade every few minutes.