Combine the soy sauce, maple syrup and horseradish in a large heavy-duty zip bag; add the tuna to the bag. Turn to cover the tuna in the marinade. Seal and chill for 1 hour, turning occasionally. Remove the tuna from the bag; discard the marinade.
Preheat your grill to high heat. Grill the tuna, covered, over high heat for 2 minutes on each side or to desired degree of doneness.
Notes
If you are preparing this recipe as gluten-free, just be sure to use brands of soy sauce and maple syrup that are known to be GF.