In a saucepan, combine the soy sauce, oil, garlic salt, pepper, brown sugar, mustard powder, ginger and pineapple juice. Bring to a boil, stirring to dissolve the sugar. Boil for about 5 minutes, then remove from the heat and allow to cool completely. When cooled, place the chicken into a large resealable bag and pour the marinade over it. Press out as much air as you can, seal and refrigerate for at least 2 hours to marinate.
Preheat a grill for medium-low heat. When hot, lightly oil the grate.
Place the chicken on the grill and discard the marinade. Cook until chicken juices run clear, 20 to 25 minutes. Turn as needed to keep the chicken from burning.
