In a large bowl whisk together the soju and the ginger juice. Add the chicken, submerge and let sit in the soju and ginger marinade for 15 minutes.
Meanwhile, in another large bowl whisk together the soy sauce, sesame oil, sugar, gochugaru, gochujang, scallion, garlic and black pepper. Reserve 2/3 of the marinade in a small bowl.
Once 15 minutes have passed, transfer the chicken to the large bowl with the gochugaru marinade. Massage the marinade into the chicken, cover and refrigerate for at least 1 hour and up to 4 hours.
Prepare a charcoal or gas grill for medium-high heat, making sure to clean the grates and lightly slick them with a kitchen rag or brush dipped in canola oil. Once hot, place the chicken on the grill and cook, brushing with the reserved marinade, until grill marks appear, the marinade has caked onto the chicken and the chicken is cooked through (an instant-read thermometer stuck into the thickest part should read 165º F), about 5 minutes per side. Transfer to a serving platter.
Serve warm with any combination of the optional sides.