Alabama BBQ Chicken on a Gas Grill

Name: Alabama BBQ Chicken on a Gas Grill
Prep time: 30 min
Cook time: 1 h
Total time: 1 h 30 min
Cuisine: Southern
Ingredients: 3/4 cup mayonnaise; 2 tablespoons cider vinegar; 2 teaspoons sugar; 1/2 teaspoon prepared horseradish; 1/2 teaspoon table salt; 1/2 teaspoon ground black pepper; 1/4 teaspoon cayenne pepper; 1 teaspoon table salt; 1 teaspoon ground black pepper; 1/2 teaspoon cayenne pepper; 2 whole chickens (3 1/2- to 4-pound), patted dry and split (see photos); Vegetable oil for grill grate