Peel grapefruit generously so that the white skin is completely removed. Cut out the flesh between the parting skins, collecting the juice. Clean and wash the lettuce and chicory and cut into bite-sized pieces. Cut the avocado in half and remove the stone.
Peel and slice the avocado. Mix grapefruit juice, vinegar, salt, pepper and sugar. Whip the oil into the mixture. Mix the salad ingredients with the marinade and arrange in a bowl