Drain grapefruit in a colander over a bowl, reserving 1/2 cup juice. Combine reserved 1/2 cup juice, cilantro, and next 4 ingredients (cilantro through salt), stirring with a whisk.
Arrange 2 cups spinach on each of 4 plates. Top each serving with 1/2 cup grapefruit sections, 3/4 cup chicken, and 2 avocado wedges. Drizzle 2 tablespoons dressing over each serving.