Combine garlic, ginger, citrus juice, soy sauce, vinegar, and sugar in a medium bowl. Place chicken halves in a large resealable plastic bag and pour in marinade. Seal bag, pressing out any air. Chill at least 12 hours and up to 2 days.
Place rack in top third of oven; preheat to 425°F. Remove chicken from bag; discard marinade. Pat chicken dry with paper towels; season lightly with salt.
Roast chicken on a foil-lined rimmed baking sheet until deep brown and cooked through, 30–40 minutes.