Combine vinegar, gin, olive oil, salt, sugar, and pepper. Skin, bone, and cube the chicken.
Marinate in dressing for 10 to 15 minutes. Peel avocados and cut into wedges; season to taste with salt pepper, and lemon juice.
Spoon marinated chicken on each plate; decorate with wedges of avocado, tomato slices, dollops of sour cream or creme fraiche, and watercress.