In a shallow dish whisk together the lemon juice, the gin, the oregano, the salt, the sugar and pepper to taste, add the oil in a stream, whisking, and whisk the marinade until it is emulsified. Add the chicken, coating it well with the marinade, and let it marinate, covered and chilled, for 20 minutes. Grill the chicken, reserving the marinade, on an oiled rack set about 6 inches over glowing coals for 7 minutes on each side, or until it is cooked through.