Combine soy sauce, maple syrup, garlic, ginger, pepper, and salt in a shallow container with a tight-fitting lid. Place salmon, flesh-side down, in the container and seal. Marinate in the refrigerator for 30 minutes.

Preheat an outdoor grill for high heat and lightly oil the grate. Once heated, turn down one side to low heat.

Place salmon, skin-side down, over low heat on the preheated grill and close the lid. Allow to cook, basting once with reserved marinade, until easily flaked with a fork, about 20 minutes. Salmon will easily remove from the grill by sliding a spatula between salmon and the skin.