1. Peel and section grapefruit over a bowl; squeeze membranes to extract juice. Set sections aside; reserve 3 tablespoons juice. Discard membranes.
2. Combine juice, olive oil, sugar, salt, and pepper, stirring with a whisk. Divide lettuce evenly among 4 plates; sprinkle 1 tablespoon crumbled feta cheese and 1 tablespoon toasted walnuts over each salad. Divide grapefruit sections evenly among salads, and drizzle with vinaigrette.