In a large bowl, whisk together the ketchup, soy sauce, brown sugar, rice vinegar, ginger, and garlic until well combined. Reserve 1⁄3 cup of the mixture for later. To the remaining mixture, add the chicken and stir to evenly coat it. You can transfer this mixture with the chicken to a gallon- size ziplock bag or simply cover the bowl with plastic wrap. Refrigerate overnight or for at least 8 hours, turning the chicken at least once.
After marinating the chicken, oil your grill grates well. Heat the grill to medium and grill the chicken for 5 to 7 minutes on each side, until cooked through, basting it with the reserved marinade after you turn it. Serve immediately.